API to change avatar

[Johennes]

For #1542, we'll need an API to set and update the user avatar.

@sandhose said on #1491:

Uploading the avatar is really tricky from MAS perspective right now, and we don't have a good way to interact with the regular C-S API from the My Account UI.

To add the required APIs on the MAS side, the quickest way would be to be able to call the C-S API as any user with the admin token MAS has to communicate with Synapse. I opened an issue on their side for this: matrix-org/synapse#16129

[sandhose]

Turns out, the approach I was going to use will not work with the Matrix Media Repo, which means it wont work in some deployments. Either I find another approach, or implement forwarding the query parameter in MMR: matrix-org/synapse#16132

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#1543