Support for guest access mode

[matrixbot]

This issue was originally created by @hughns at matrix-org/matrix-authentication-service#1445.

MSC2967 proposes a guest scope urn:matrix:client:api:guest which is supported by Synapse.

However, MAS currently doesn't provide a way for a guest token to be issued.

[bartvdbraak]

I just deployed MAS on our staging environment at Blender and noticed that guest accounts were unsupported. Unfortunately, for our usecase this is currently a dealbreaker.

Hope to see updates on this topic soon :)

[4www]

Missing guest users also breaking https://gitlab.com/sctlib/libli and https://gitlab.com/sctlib/mwc, rendering both projects obsolete (currently depending on matrix.org homeserver to issue guest user tokens and allow un-registered user to browse public rooms like public social network profiles).

[A5rocks]

MSC2967 removed urn:matrix:client:api:guest before being merged. See matrix-org/matrix-spec-proposals@f65aef3

@sandhose I don't see any discussion of this in the PR, is there a reason? Just to make the proposal simpler? Do you have recommendations on how guest accounts should work nowadays?