Add IRedisConnectionProvider interface

[jcreus1]

Hi @slorello89. I'd like to extract an interface for the RedisConnectionProvider class, to allow mocking/substitution for unit testing.

My use case is that I have an ASP.NET Core Web API project injecting a RedisConnectionProvider object into my caching implementation class:

Program.cs

Caching Implementation:

I am currently unable to unit test this class, as our chosen mocking tool (NSubstitute) requires either an interface or a concrete implementation containing virtual methods it can override. I would therefore like to create an IRedisConnectionProvider interface and to update each reference to RedisConnectionProvider to use this interface.

Will raise a PR with a simple implementation.