Refactor/Unify Build and Graph OM and API

[JanKrivanek]

Prerequisity for indicating Build Submissions in Build Events (which will be needed by BuildCheck to detect Restore)

Context

Build and Graph Build OM and API is largely duplicated. Building on top of this (e.g. to add 'BuildSubmissionStartedEventArgs') would require even further duplication of code.

So let's deduplicate the OM and API.

Changes Made

• Unify Request, Result, Submission
• Apply nullability checking on all refactored code
• Unify the functionality in BuildManager
• Apply nullability checking on whole BuildManager

Important notes:

• I sealed the existing OM - letting it open was a design mistake that doesn't have sane usage. Hopefully there is none - we can easily revert if there proves to be some usages.

• For some reasons the Results Cache is applicable only for 'standard' build requests - not for graph build requests. I do not fully understand why. I have left that aspect untouched
  

  msbuild/src/Build/BackEnd/Components/Caching/IResultsCache.cs

  Line 33 in 9bea802

  BuildResult GetResultForRequest(BuildRequest request);

• ExecuteSubmission differs very significantly for BuildRequest and GraphBuildRequest - those are left in their separate functions (but at least there is a single entrypoint function to them)

• The entrypoint location of where we should introduce emiting of BuildRequestStarted is indicated in code by TODO with some hints (https://github.com/dotnet/msbuild/pull/10172/files#diff-2b0716a511d8f4ee690ebd5c3a59dec1e3f9a5eab4ab2a80a1018820a658accbR1370-R1372) - so once this PR is merged, there hopefully should be a no blocker for Analyzers prototyping - Detect and skip restore (evaluation and execution) #9747 (FYI @maridematte)

FYI @dfederm - this is heavily touching graph requesting - so your PoV would be very helpful

[JanKrivanek]

/azp run

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[JanKrivanek]

rekicking CLA bot

[JanKrivanek]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[JanKrivanek]

rekicking cla

[maridematte]

It looks good to me, but because there are so many changes to just nullable variables it gets harder to review this. Maybe all the nullable changes could have been in another PR just so we can focus on the functionality of the refactor.

[JanKrivanek]

Experimental VS insertion: https://devdiv.visualstudio.com/DevDiv/_git/VS/pullrequest/562572 (build goes fine now!)