Add Evaluate(Ref) Gateway API

[jedevc]

This PR is a replacement for #2947 (cherry-picking that commit for consistency, so that calling Evaluate over each ref in a result is the same as setting Evaluate: true in the SolveRequest).

This can be invoked on a ref using ref.Evaluate() in a frontend - this allows for on-demand unlazying a reference, instead of needing to wait until export. Previously, as a hack, this could be done using StatFile, however this has the disadvantage of needing to pull the image to be able to mount it, which isn't required to ensure that the image is unlazied.

Additionally, the previous Evaluate field in the SolveRequest was not sufficient, as this causes the Solve to block - we may want to perform other operations, before then determining whether to Evaluate, as in docker/buildx#1197.

[tonistiigi]

Not part of your PR, but why does getImmutableRef() take a path ? 🤯

[jedevc]

👀 seemingly to print a deceiving error message apparently...

func (lbf *llbBridgeForwarder) getImmutableRef(ctx context.Context, id, path string) (cache.ImmutableRef, error) {
	lbf.mu.Lock()
	ref, ok := lbf.refs[id]
	lbf.mu.Unlock()
	if !ok {
		return nil, errors.Errorf("no such ref: %v", id)
	}
	if ref == nil {
		return nil, errors.Wrap(os.ErrNotExist, path)
	}

Technically it's possible for a value in Result.Ref to be nil, but printing file does not exist: <path> for that doesn't feel like the right message - empty ref feels like a more apt description of what's gone wrong. Definitely an edge case, a sane frontend should never produce nil refs like that.

I'll add a commit on top of the stack in this PR to tidy up the method, good catch.