Add missing APIs to contract customization

[eiriktsarpalis]

Fix #71123. Contributes to #63686.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Fix #71123. Contributes to #63686.

Author:	eiriktsarpalis
Assignees:	-
Labels:	area-System.Text.Json, new-api-needs-documentation
Milestone:	-

[krwq]

There are couple of things left here:

• fix functional tests to use patterns we recommend to users (i.e. whenever calling Properties.Clear() we should be creating types from scratch and combining)
• we should not pre-sort properties for reflection
• allow users to fix extension data on properties of wrong type/attribute on two properties + functional tests
• fix docs on GetConverter w.r.t null/throw and make sure we match behavior in GetTypeInfo

I'm ok with fixing that later but we need to track remaining work somewhere. Otherwise looks good

[eiriktsarpalis]

There are couple of things left here:

• fix functional tests to use patterns we recommend to users (i.e. whenever calling Properties.Clear() we should be creating types from scratch and combining)
• we should not pre-sort properties for reflection
• allow users to fix extension data on properties of wrong type/attribute on two properties + functional tests
• fix docs on GetConverter w.r.t null/throw and make sure we match behavior in GetTypeInfo

I'm ok with fixing that later but we need to track remaining work somewhere. Otherwise looks good

As a matter of record, I will reiterate that these concerns have either been addressed, need not be addressed in the scope of this change or will not be addressed by design. Individual remarks elaborate these points further. This PR is ready for sign-off.

[jeffhandley]

While there are still some open discussions on this, each of them that was agreed to be addressed later has a backing issue and doesn't affect the raw functionality as best as I can tell. Let's go ahead and get this in ahead of the weekend.