Idea: Add support for async AuthenticationServices on server

[Daniel-Svensson]

Currently the IAuthentication interface needs to be implemented by "AuthenticationServices", however it is synchronous.

It would be nice to have a a similar interface "IAuthenticationAsync" /"IAsyncAuthentication" which returns Tasks instead.

• Should work at runtime to have either interface implemented
• It should work the same for code generation (i.e. the code generation detect that it is an authorization service)

[Daniel-Svensson]

Have a look at the test project and aspnetcore sample for examples of how the methods can be implemented asynchronously (public async methods while having explicit implementation of sync interface methods)