support customBuiltins parameter in loadPolicy

[abrgr]

Hi,

This PR adds support for custom builtins by adding an additional parameter to loadPolicy, somewhat related to the discussion in #23.

One potential point of contention is that this always favors first-party builtins over provided builtins.

P.S. opa is fantastic - great work!

[srenatus]

Thanks you this is a great contribution 👏

[udnes99]

This contribution is greatly appreciated!
I have one question about providing async functions--Is it possible?
For example, this would be needed to implement a function to make HTTP calls.

I tried providing an async function when loading the rego used for testing here; simply made the provided function to custom.zeroArgBuiltin async.
It does not seem to work, and the result is not the expected string, but an empty object.

[srenatus]

I have one question about providing async functions--Is it possible?

I don't think so. For one thing, to have wasm interactions work with anything async would mean to either process the wasm module with emscripten's asyncify, or deal with it manually, see this blog post.