feat: integrate Element with Synapse REST auth provider

[liviaUeno]

REST Auth Provider for Matrix Synapse

This module implements a custom password authentication provider
for Matrix Synapse using an external REST API (paywall / authentication service).

• authenticate users against an external service
• automatically create Matrix users on first successful login
• generate Matrix user IDs based on external user data

This module does not expose a registration endpoint.
User creation happens implicitly during login if the user does not exist.

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Failed authentication

Authentication will fail if:

• credentials are invalid (non-200 HTTP response), or
• required fields (email, id) are missing or malformed
  In these cases, Synapse will reject the login attempt.

Matrix user creation

If authentication succeeds and the Matrix user does not exist:

• the user is created automatically
• the Matrix localpart is generated as follows:
  • first_last if both first and last name are present
  • otherwise, the email local part (before @)
• the localpart is normalized to:
  • lowercase
  • ASCII characters only
  • non-alphanumeric characters replaced with _