Make output assemblies compatible with link-level trimming

[brantburnett]

This will only work when using System.Text.Json with a JsonSerializerContext, but should be doable in that case. However, we have a lot of rework to do for compatibility.

• [x] Eliminate or annotate the use of reflection for literal serializers
• [x] Eliminate or annotate the use of reflection for multipart serializers
• [x] Apply any other missing trimming annotations
• [x] Implement the use of JsonSerializerContext with System.Text.Json
• [x] Apply the trimmable assembly attribute, but only if all extensions advertise compatibility with trimming

There are also some optional components that could be implemented as well. These could improve trimming effectiveness.

• [ ] Use declarative serializer discovery in code rather than registering all serializers in a dictionary

[brantburnett]

At this time splitting the JsonSerializerContext may be a bridge too far. The implementation is internal so we can still come back and implement that in a future version.

This is particularly supported by this comment: https://github.com/dotnet/runtime/issues/101663#issuecomment-2107522551 which makes it appear that JsonSerializerContext may not be the long-term solution at all.