Add optional support for GetAllAsync() via HttpRemoteStore

[Blackburn29]

I ran into this issue while converting from an EF store to HTTP in a project. My comment on issue https://github.com/Finbuckle/Finbuckle.MultiTenant/issues/808 mainly provides the need for this and I managed to implement it manually within my app, however I'd like to mainline this for future projects. I can definitely understand why people would not want this, which is why I based the implementation off the existing route template and will throw NotImplementedException in the case of the external http service having an undefined route for this implementation.

Feedback is greatly appreciated!