Provide the capability to add an in-memory secret provider in the secret store for testing

[stijnmoreels]

Is your feature request related to a problem? Please describe.
When people are trying to test their code, and want to check some (non-secret) values from being used by their code. It may be an option to include also an in-memory secret provider, possible in separate project and with the 'testing' namespace to make sure that people understand that this is for testing.

Describe alternatives you've considered
Users can create their own implementation, but a simple build-in in-memory instance is still useful then.

Additional context
Original posted here: arcus-azure/arcus.security#256