Support for authenticated media

[kfiven]

After the landing of matrix-org/matrix-spec-proposals#3916 the profile picture of mxid does not load.

Example: https://matrix.to/#/@ukiradante:matrix.org

[t3chguy]

This will be tricky given that matrix.to doesn't actually have any credentials to authenticate with, it relies on unauthenticated APIs exclusively.

[ShadowRZ]

Giving credentials doesn't seems correct to me, as a credential would only apply to a single homeserver while matrix.to supports using other servers to get the preview.

Also IMO using credentials in matrix.to would complicate the implmentation of matrix.to, as to protect the credential, it should use a server side request to the authenticated endpoints, thus requiring the matrix.to to also have a private endpoint to fetch media itself, very similar to what some bridges should do.