AWS - [Security] Don't throw exception with secret value in message

[davidsloan]

This is a rebase/rework of #23 by @adrianisk against the latest version of the code base.

Original PR description:

Summary

This change prevents the library from throwing an exception that contains the secret text in the exception message when using the AWS SecretsManager provider. While doing some local testing I noticed the thrown exception, along with the secret, will end up in Kafka Connect's logs which isn't great. To fix I updated the code to catch the JsonParseException which contains the secret value, and throw an exception with a generic parsing error message in its place.

Test

I added a unit test to verify, prior to the fix it failed with

- should not throw exception with secret value in message *** FAILED *** (53 milliseconds)
  "Unrecognized token 'secret': was expecting (JSON String, Number, Array, Object or token 'null', 'true' or 'false')
   at [Source: (String)"secret-value"; line: 1, column: 7]" included substring "secret-value" (AWSSecretProviderTest.scala:144)