Support secrets other than in json format

[techgaun]

AWS secrets manager does not mandate JSON or key-value pair value for the secrets value (although recommended). However, in the implementation, there's explicit json decoding at https://github.com/gordalina/hush_aws_secrets_manager/blob/a76776d/lib/provider.ex#L33

This causes issues with cases where one would like to store non-json values such as plaintext string or TOML, etc. Is it possible to remove that or support passing a custom function to execute for parsing/decoding instead?

A better approach might be to just retrieve the data in provider and not handle the data format at all. The transformers then can be implemented to handle the data accordingly.

[gordalina]

JSON decoding should definitely be removed from the provider and used as a transformer. Feel free to submit a PR! If not I'll work on this in a couple of weeks.