[federation] Add event auth endpoint

[matrixbot]

This issue was originally created by @APwhitehat at matrix-org/dendrite#482.

endpoint: GET and POST /_matrix/federation/v1/event_auth/{roomID}/{eventID}.

Phase: 1 (still incomplete ?)

ref: https://github.com/matrix-org/synapse/blob/master/synapse/federation/federation_server.py#L342
https://github.com/matrix-org/synapse/blob/master/synapse/handlers/federation.py#L935

[matrixbot]

This comment was originally posted by @APwhitehat at matrix-org/dendrite#482 (comment).

self note: This is fairly straightforward after #485 is merged.

[matrixbot]

This comment was originally posted by @sudsj at matrix-org/dendrite#482 (comment).

Hi, I would like to work on this issue.:)

[matrixbot]

This comment was originally posted by @APwhitehat at matrix-org/dendrite#482 (comment).

Fixed in #905, I believe. @kegsay ?

[matrixbot]

This comment was originally posted by @kegsay at matrix-org/dendrite#482 (comment).

Yep