bake: allow interception to create derived contexts

This patch allows high level clients to define an EvalContext method
which can derive a new context given a block and the base parent
context.

This allows users of the package to intercept evaluation before it
begins, and define additional variables and functions that are bound to
a single block.

Signed-off-by: Justin Chadwell <me@jedevc.com>

bake/hclparser/stdlib.go

@@ -124,3 +124,11 @@ var timestampFunc = function.New(&function.Spec{
 		return cty.StringVal(time.Now().UTC().Format(time.RFC3339)), nil
 	},
 })
+
+func Stdlib() map[string]function.Function {
+	funcs := make(map[string]function.Function, len(stdlibFunctions))
+	for k, v := range stdlibFunctions {
+		funcs[k] = v
+	}
+	return funcs
+}