Support arbitrary JSON format for config coming from AWS Secrets Manager

[andrus]

Currently AwsSecretFactory supports secrets format that is a JSON object with no nesting (i.e. {"a":"b","c":"d"}). No arrays or nested JSON structures are supported. This assumption works well for standard AWS secret choices (RDS, etc.), but severely limits custom configurations. Still need to support "mergePath" of course.

As a part of implementation we need to create an alternative to InPlacePropertiesMerger (currently used by AwsSecretFactory) that supports proper JSON merging.