[dotnet] parametric app improvements

[lucaspimentel]

Motivation

Improve the experience of modifying, running, and troubleshooting the parametric app for dotnet.

Changes

• nuget packages (TODO: move this into a separate PR?)
  • pick up local nuget packages dropped in utils/build/docker/dotnet/parametric/ (TODO: support binaries/)
  • prioritize local packages over the nuget.org feed
• JSON serialization
  • use the built-in System.Text.Json, remove dependency on Newtonsoft.Json
  • de-deduplicate code into helper methods, add shared validation and logging
• small reflection refactoring to reduce repetition
• enable more built-in logging by default
• misc code clean-up to reduce compiler warnings
  • add missing nullability annotations and null checks
  • make members private where possible

Sidenote: while making these changes I found several unrelated bugs when using the .NET tracing library's manual API

• setting Span.Service = null after the span is created causes an exception when the span is closed
• using SpanCreationSettings.Parent = SpanContext.None results in a 64-bit trace id (the upper 64 bits are truncated somewhere)
• passing a delegate to SpanContextExtractor that returns null causes an exception. Working on a fix here: [tracing] add null checks when iterating headers in propagators dd-trace-dotnet#6460

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes (if something not related to your task is failing, you can ignore it)
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner. We're working on refining the codeowners file quickly.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• If PR title starts with [<language>], double-check that only <language> is impacted by the change
• No system-tests internal is modified. Otherwise, I have the approval from R&P team
• CI is green, or failing jobs are not related to this change (and you are 100% sure about this statement)
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added (or removed)?
  • Get a review from R&P team