Fix container registry creation and support updating subscription plans.

[andrewsomething]

When registry support went into general availability, it added the requirement of specifying a subscription plan. This adds the subscription_tier_slug to both the resource and data source allowing it to be specified on create and updated.

$ make testacc TESTARGS=-run='ContainerRegistry'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=ContainerRegistry -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDataSourceDigitalOceanContainerRegistry_Basic
=== PAUSE TestAccDataSourceDigitalOceanContainerRegistry_Basic
=== RUN   TestAccDigitalOceanContainerRegistry_importBasic
--- PASS: TestAccDigitalOceanContainerRegistry_importBasic (19.08s)
=== RUN   TestAccDigitalOceanContainerRegistryDockerCredentials_Basic
--- PASS: TestAccDigitalOceanContainerRegistryDockerCredentials_Basic (17.92s)
=== RUN   TestAccDigitalOceanContainerRegistryDockerCredentials_withExpiry
--- PASS: TestAccDigitalOceanContainerRegistryDockerCredentials_withExpiry (17.73s)
=== RUN   TestAccDigitalOceanContainerRegistry_Basic
--- PASS: TestAccDigitalOceanContainerRegistry_Basic (20.75s)
=== CONT  TestAccDataSourceDigitalOceanContainerRegistry_Basic
--- PASS: TestAccDataSourceDigitalOceanContainerRegistry_Basic (22.86s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean    98.358s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/setutil        (cached) [no tests to run]

[bentranter]

LGTM 👍