Prevent misuse of IdentityProviders

[josephdecock]

Sometimes when users have tried to implement dynamic providers with custom stores, there's a confusion about the intended usage of IdentityProvider vs its derived types.

@josephdecock maybe we could do better in the code to detect this type of thing? and maybe emit more helpful logs?

Originally posted by @brockallen in https://github.com/DuendeSoftware/Support/issues/556#issuecomment-1466469687

[josephdecock]

First thought - make IdentityProvider abstract (obviously that's a breaking change though)

[brockallen]

Our first attempt at ensuring people follow the right pattern was via our sample (WS-Fed one IIRC). Is that not enough?