Implement all properties for TokenIntrospectionResponse optional claims

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As described by RFC 7662 - OAuth 2.0 Token Introspection

(Taking over IdentityModel/IdentityModel#577 to the new repository, adapted to the new Duende.IdentityModel namespace.)

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🎉 All tests are finally green ✅!

[josephdecock]

I'd like to remove the use of Lazy in this PR. That way, developers can make their own choice about if they need Lazy access to these properties. My guess is that in practice, there wouldn't be much gained by not evaluating the properties when they're not needed, and depending on access patterns, the allocations needed by Lazy and the locking it does to provide thread safety might introduce other overhead. If we leave it out of the library, then individual applications can always add whatever optimizations they want.

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Fair point. Addressed in f75f651.

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I just rebased this pull request on the main branch and resolved the conflicts that arose from #86.

I also slightly modified how the exp, iat and nbf claims are parsed in c01e68b to avoid throwing in case those contain an invalid number. With the previous Lazy implementation it would not throw before accessing them.