Removes config.fake_kms_api and replace with config.kms_client, which can be passed any Client object, including the faked client.

See the updated docs for more information.

Adds support for arn_prefix in configuration. See README.md for more details.

Move minimum Ruby required to 2.5. Address depreciation warnings in Ruby 2.7.

Switch to using AWS SDK v3 and aws-sdk-kms.

Makes validation of real column's non-existence occur on first use, not on attribute definition.

Fixes bugs described in #1 and #2