[csharp][generichost] Use TypeInfoResolver

[devhl-labs]

The initial source generation implementation has some issues. Constructing the JsonSerializerContext using the JsonSerializerOption copy constructor (below) does not work. The JsonConverters do not get copied which means many de/serializations will not work.

public ActivitySerializationContext(JsonSerializerOptionsProvider optionsProvider): base(new(optionsProvider.Options))

Additionally, dealing with both a JsonSerializerOption and JsonSerializerContexts is nearly impossible to handle in my other PR to handle all responses. This change consolidates to only the JsonSerializerOption by making the JsonSerializerOption aware of the JsonSerializerContexts by using the TypeInfoResolver, which is only available in .Net 7. The source generator option was only added very recently, so it should not be a big breaking change.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
• File the PR against the correct branch: master (7.0.1 - patch release), 7.1.x (minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.