Exchange token for proxy

- [ ] Implement an DB class to get a proxy from the `ProxyDB`
- [ ] Add a route to return the Proxy `GET /auth/proxy`
- [ ] Implement the Proxy delegation mechanism locally to have a proxy lasting as long as the refresh token with the same group. Check that the properties in the token matches those defined in the CS for the said group. (@chaen to provide a standalone script to perform delegation)
- [ ] Add the VOMS extension (voms-proxy-fake is fine for now)
- [ ] Call the route from the cli

The proxy delegation mechanism is documented here https://github.com/DIRACGrid/DIRAC/blob/4b5a8216da61ef5cb07e53f2e3a5ad2b4f01aad6/src/DIRAC/Core/Security/test/Test_X509Chain.py#L508-L570