Pass context.Context to cue build, evaluation and builtins

[bozaro]

Is your feature request related to a problem? Please describe.
I want have ability to solve two problems:

• allow interrupt cue evaluation (unfortunately it can take minutes);
• pass some data to builtins functions (in my case I try create request cache for gRPC client calls).

Describe the solution you'd like
I wan't to set context.Context to every cue evaluation operation.

Something like this: master...joomcode:cue:context-poc

func EvalWithContext(ctx context.Context, value cue.Value, args ast.Expr) cue.Value {
        return value.
                WithContext(ctx).
                FillPath(cue.MakePath(cue.Str("args")), args).
                LookupPath(cue.MakePath(cue.Str("result"))).
                Eval()
}

[mvdan]

Indeed this is something we should add. See #1104; I'm going to close this for now, to prevent the discussion from being split :)