First iteration of NestedSet-on-Skyframe

Main Idea:
When an Action is executed with ActionExecutionFunction, the actions's input
NestedSet<Artifact> could be evaluated as an ArtifactNestedSetKey [1].
ArtifactNestedSetFunction then evaluates the ArtifactNestedSetKey:
- Evaluates the directs elements as Artifacts. Commit the result into artifactToSkyValueMap.
- Evaluates the transitive elements as ArtifactNestedSetKey.

ActionExecutionFunction can then access this map to get the Artifacts' values.

[1] Heuristic: If the size of the NestedSet exceeds a certain threshold, we evaluate it as an ArtifactNestedSetKey.

Notes:
- The new feature is hidden behind a flag --experimental_nested_set_as_skykey_threshold. The default code path is not affected. We'd like to get this checked into the codebase, then from there improve it incrementally.
- It currently only works without the Action Cache.

Next Steps:
- Figure out the main source of additional pressure on GC & address it
- Add tests

RELNOTES: Introduce --experimental_nested_set_as_skykey_threshold
PiperOrigin-RevId: 273938957