Update GenerateTokenContext to expose SecurityTokenDescriptor and adopt the new URI-style token type identifiers in the client, core, server and validation stacks

[kevinchalet]

This PR:

• Updates the GenerateTokenContext class to allow tweaking the SecurityTokenDescriptor instance before IdentityModel is invoked to create the token, which can be used to override the creation/expiration dates or the token type.
• Updates a few handlers to not throw when a token type isn't recognized (instead, no mapping is done and the value is flowed as-is). It's something that needed to be changed for OAuth 2.0 token exchange to work with tokens that are not issued by the OpenIddict server itself.
• Implements the strategy described in that thread: Adopt the new URI-style token type identifiers in the client, core, server and validation stacks #2296.