[Deliverable] Standalone RLN membership allocation service module

[jm-clius]

Owner: AnonComms Zerokit-RLN

FURPS:

• F1. An RLN membership allocation service can register ID commitments on behalf of third parties
• F2. The RLN membership allocation service has a pluggable authentication mechanism to determine eligibility for membership
• F4. The RLN membership allocation service can run as a standalone module or mounted on existing modules

Checklist:

• Specs: link to specs and/or API definition
• Code: link to GitHub issues/PRs/Epic
• Dogfood: link to dogfooding session/artefact
• Docs: links to README.md or other docs