ast: extend capabilities to return min compatible version

This commit adds the ability to determine the minimum compatible OPA
version for a set of capabilities. This can be coupled with the
capabilities generated by the compiler to determine the min. compatible
version of a policy/bundle. The build has been extended to generate the
version index that lets us quickly check the required version for each
builtin/feature/keyword in the capabilities.

Signed-off-by: Torin Sandall <torin@styra.com>