Factor out `MatrixFederationAgent`

Sydent: sydent/sydent/http/matrixfederationagent.py
synapse: synapse/synapse/http/federation/matrix_federation_agent.py

And possibly these two in Synapse too?

```
synapse/tests/http/test_proxyagent.py
211:class MatrixFederationAgentTests(TestCase):

synapse/tests/http/federation/test_matrix_federation_agent.py
64:class MatrixFederationAgentTests(unittest.TestCase):
```