topdown/builtins: Add object support for GraphQL builtins. (#4752)

This commit adds support for AST objects to be usable in place of
strings for several of the GraphQL built-in functions, to improve
the composability of the GraphQL set of built-ins, and to dramatically
reduce the amount of redundant parsing when writing GraphQL policies.

Fixes: #4742

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>