All: Consider using generic type for key in store/load

The `store` and `load` methods in the `AppEncryption` interface currently only operate with a single `String` key. Consider changing to be a generic type that would give more flexibility.

Simple enough to do for Java and C# but not sure what other languages' support for this would look like.

Start by locating the AppEncryption interface and its store and load declarations in the C#, Java, and other language implementations. Compare each language's support for generic key types and determine the desired cross-language API behavior; done is not defined because the issue names no target types, files, or tests.

Written by the indexing model from the issue text.