Add ClientRegistration.clientSettings.requireProofKey to Enable PKCE

[kse-music]

Closes gh-16382

[rwinch]

Thanks for the PR @kse-music I'm putting this on pause as the team discusses the underlying ticket.

PS: Sorry for not flagging the ticket for team discussion until after you had already created the PR.

[rwinch]

@kse-music Thanks again for the PR!

After speaking with @jgrandja offline, he suggested the property ClientRegistration.clientSettings.requireProofKey=(true|false) to align with Autorization Server's RegisteredClient.[clientSettings](https://github.com/spring-projects/spring-authorization-server/blob/1.4.1/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/client/RegisteredClient.java#L73).requireProofKey. In this case, we can introduce the ClientSettings in the same package as ClientRegistration and it does NOT need to be backed by a Map or extend AbstractSettings. If needed, we can later introduce AbstractSettings and back it by a Map at that point.

Also, I would expect that this PR enable PKCE for that RegisteredClient if the new property is set to true.

[rwinch]

Thanks for the Pull Request! This is now merged into main via the merge commit 4fc99aa 😄

I included the original commit as 8d3e084 that was rebased along with additional polish:

• 85d7cc1 Document requireProofKey
• 004f386 Move ClientSettings to ClientRegistration
• 4c53356 Ensure missing ClientRegistration.clientSettings JSON node works
• f9498d3 PKCE cannot be true and AuthorizationGrantType != AUTHORIZATION_CODE
• ab629cc Add AuthorizationGrantType.toString()
• b0a4dcb ClientSettings equals, hashCode, toString
• 2665a92 Ensure that ClientSettings cannot be null
• 0ed7b18 DefaultServerOAuth2AuthorizationRequestResolver requireProofKey support
• 8d3e084 Add ClientRegistration.clientSettings.requireProofKey to Enable PKCE