Support custom Json serde for SecretsManager

[samu-developments]

Thanks for this great library!

We had a use case where we needed to deserialize a json with a number value (not a quoted number string) from AWS secretsmanager, eg:

{
  "port": 5432,
  "username": "test",
  ...
}

To do this one need to set the isLenient field to true in Json builder:

val json = Json { 
  isLenient = true 
}

This PR adds Json as a constructor parameter to AwsSecretsManagerPreprocessor to support custom json serde config. Let me know what you think.

(In the end we just added our own custom preprocessor, which also works fine)

[sksamuel]

Couple of test failures. Will merge once green.

[samu-developments]

Couple of test failures. Will merge once green.

Local tests were loading region from local config, works now. Thank you!