apps: Suppress diff when env type is GENERAL.

[andrewsomething]

The API does not consistently return "type":"GENERAL" when set for an environment variable. This leads to changes shown on each run if an env var type is set to GENERAL. As being unset and being set to GENERAL are functionally, the same, we can safely ignore the diff.

Resolves one issue mentioned in #514

Test failure with the test change adding type = "GENERAL":

$ make testacc TESTARGS="-run='TestAccDigitalOceanApp_Envs'"==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run='TestAccDigitalOceanApp_Envs' -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanApp_Envs
=== PAUSE TestAccDigitalOceanApp_Envs
=== CONT  TestAccDigitalOceanApp_Envs
    resource_digitalocean_app_test.go:189: Step 3/3 error: After applying this test step and performing a `terraform refresh`, the plan was not empty.
        stdout
        
        
        An execution plan has been generated and is shown below.
        Resource actions are indicated with the following symbols:
          ~ update in-place
        
        Terraform will perform the following actions:
        
          # digitalocean_app.foobar will be updated in-place
          ~ resource "digitalocean_app" "foobar" {
                active_deployment_id = "e80ef063-1442-4ff7-84cd-c25b4e9d0e05"
                created_at           = "2020-11-05 16:34:37 +0000 UTC"
                default_ingress      = "https://tf-acc-test-tpph1rync4-ouyzo.ondigitalocean.app"
                id                   = "28767f1d-fd64-489a-9381-b6f8a17f8e4c"
                live_url             = "https://tf-acc-test-tpph1rync4-ouyzo.ondigitalocean.app"
                updated_at           = "2020-11-05 16:39:40 +0000 UTC"
        
              ~ spec {
                    domains = []
                    name    = "tf-acc-test-tpph1rync4"
                    region  = "ams"
        
                  ~ service {
                        environment_slug   = "go"
                        http_port          = 8080
                        instance_count     = 1
                        instance_size_slug = "basic-xxs"
                        name               = "go-service"
        
                      - env {
                          - key   = "FOO" -> null
                          - scope = "RUN_TIME" -> null
                          - value = "baz" -> null
                        }
                      + env {
                          + key   = "FOO"
                          + scope = "RUN_TIME"
                          + type  = "GENERAL"
                          + value = "baz"
                        }
        
                        git {
                            branch         = "main"
                            repo_clone_url = "https://github.com/digitalocean/sample-golang.git"
                        }
        
                        routes {
                            path = "/"
                        }
                    }
                }
            }
        
        Plan: 0 to add, 1 to change, 0 to destroy.
--- FAIL: TestAccDigitalOceanApp_Envs (310.77s)

All Apps tests pass after the DiffSuppressFunc is added:

$ make testacc TESTARGS="-run='TestAccDigitalOceanApp' -parallel=12"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run='TestAccDigitalOceanApp' -parallel=12 -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanApp_importBasic
=== PAUSE TestAccDigitalOceanApp_importBasic
=== RUN   TestAccDigitalOceanApp_Basic
=== PAUSE TestAccDigitalOceanApp_Basic
=== RUN   TestAccDigitalOceanApp_StaticSite
=== PAUSE TestAccDigitalOceanApp_StaticSite
=== RUN   TestAccDigitalOceanApp_Envs
=== PAUSE TestAccDigitalOceanApp_Envs
=== RUN   TestAccDigitalOceanApp_Worker
=== PAUSE TestAccDigitalOceanApp_Worker
=== CONT  TestAccDigitalOceanApp_importBasic
=== CONT  TestAccDigitalOceanApp_Envs
=== CONT  TestAccDigitalOceanApp_Basic
=== CONT  TestAccDigitalOceanApp_StaticSite
=== CONT  TestAccDigitalOceanApp_Worker
--- PASS: TestAccDigitalOceanApp_StaticSite (362.97s)
--- PASS: TestAccDigitalOceanApp_importBasic (453.35s)
--- PASS: TestAccDigitalOceanApp_Worker (654.88s)
--- PASS: TestAccDigitalOceanApp_Envs (948.30s)
--- PASS: TestAccDigitalOceanApp_Basic (1088.84s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean    1088.850s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       0.021s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/setutil        0.016s [no tests to run]

[bentranter]

LGTM!