Allow full paths in REST requests

[paulomarg]

WHY are these changes introduced?

Currently, we can't handle requests like access scopes (/admin/oauth/access_scopes.json) or token access (/admin/api_permissions/current.json), because these resources use custom paths instead of the default /admin/api/{version}/....

This means we can't easily reach those endpoints unless the code uses the base HTTP client that the REST one is built upon, which makes things unnecessarily complicated.

WHAT is this pull request doing?

This PR solves that problem by allowing REST calls in the Admin API client to be made using a full path. Therefore, requests like

const client = new RestClient(domain, 'token');
const response = await client.get({path: '/admin/some-path.json'});

work just like any regular request would.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)