Cache SDK resolver data process-wide

[ladipro]

Fixes #9302

Context

Unless a shared evaluation context is used, each project evaluation creates a fresh new SdkResolverLoader, walks the disk to discover resolver manifests, calls Assembly.LoadFrom on resolver assemblies and so on. The cost of this is non-trivial (close to 3% of total evaluation time for solution like OrchardCore) while arguably there is no way how any of this can change while an MSBuild process is running.

Changes Made

• Refactored the code to clean up unused parameters and changed several types from IList<> to IReadOnlyList<> to codify the fact that they are immutable.
• Introduced CachingSdkResolverLoader as a subclass of SdkResolverLoader and plugged it into SdkResolverService.

Testing

Existing unit test and an experimental VS insertion.

Notes

Consider reviewing commit by commit.

[JanKrivanek]

Overall looks good.

I love the extra diligence in ensuring the encapsulation!!