Feature request: API to pass KMSMasterKeyProviderConfig to key providers' KMS clients

[alex-chew]

Currently, the best way to configure the KMS clients that Master Key Providers create, is to subclass the desired provider to supply the desired KMS client configuration. This can be better handled by providing a dedicated API or argument, or to refactor the Python ESDK to use keyrings.

Context: #446

[lucasmcdonald3]

ESDK now supports keyrings, which take in a KMS client object on creation. See an example: https://github.com/aws/aws-encryption-sdk-python/blob/master/examples/src/aws_kms_keyring_example.py

It's unlikely we will take this feature request on as-is, since MKPs are now "legacy" and will be marked as deprecated soon. Keyrings are the recommended way to encrypt data.