[rush] feat(buildcache): improve access to operation build cache

[aramissennyeydd]

Summary

Work towards #3994. This adds a new method to ProjectBuildCache, forOperation that generates a build cache accessor from an operation and a few additional classes, but aims to reduce the amount of boilerplate it takes to interact with the build cache.

Details

• Moves stateHash calculation to Operation, and only sets after a calculate method is called. I was trying to mirror this as best I could to what we had before, the big issue with moving it to, say, the constructor is we'd have to add more options to allow us to calculate it. This way, we calculate both an operation and its dependencies and can cache the hash result easily on each operation itself.
• Creates a new static method forOperation on ProjectBuildCache that should now be easier to call from rush-sdk code, assuming you've called the calculateStateHash method.

How it was tested

I ran rush build a few times to make sure that caching isn't in place on changes and is in place when nothing has changed. Are there any more in depth checks I should do?

Impacted documentation

[dmichon-msft]

I like the idea, but using the wrong object to store the state.

[aramissennyeydd]

@dmichon-msft This should be ready for another round 🙇 !

[dmichon-msft]

Mostly looks good, just a couple of nits.