Add interface for IdentityServerTools

[matt-goldman]

What issue does this PR address?
IdentityServerTools is useful and I expect used in a lot of places. However as it's a concrete class it is difficult to test services that depend on it. This PR creates an abstraction IIdentityServerTools which can be injected instead, and can therefore be mocked easily for tests. This is important for testing services that depend upon IdentityServerTools but don't explicitly need it for testing purposes.

Note: This would be a breaking change. Personally I think that's ok, but if you wanted to maintain backward compatibility, we could keep both registrations:

builder.Services.AddTransient<IIdentityServerTools, IdentityServerTools>();
builder.Services.AddTransient<IdentityServerTools>();

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[brockallen]

@AndersAbel check in our docs for updates based on this: https://docs.duendesoftware.com/identityserver/v6/tokens/internal/

[brockallen]

Thanks for this @matt-goldman -- we'll work on it!

[josephdecock]

Am I right to think that this is a breaking change only in the sense that the class is marked obsolete?

[AndersAbel]

If someone has customized the service/class and replaced it with their own in DI this will revert back to using the default version of the service. I added Obsolete because I think that helps flagging this for anyone that has usage/customization.