[WIP] JS Scripting Support

[danslapman]

Fixes #14

Problem

Currently existing "pseudofunction calls" is a quite limited mechanism, which is not enough in some complex scenarios like mocking complex authorization (e.g. http-signature), math computaions and others.

Solution

This PR replaces pseudofunctions with Graal-based JavaScript sandbox

Checklist

• [ ] Unit test all changes
• [ ] Update README.md if applicable
• [x] Add [WIP] to the pull request title if it's work in progress
• [ ] Squash commits that aren't meaningful changes

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