Add ability for dendrite to read AS registration files from blueprint

[ShadowJonathan]

(I encountered this line today;

https://github.com/matrix-org/complement/blob/0e88861054556a734f40d5754d93499a7ac311c1/tests/federation_room_join_test.go#L531

So here's an issue to keep track of it.)

[ArpitKRS]

Please /assign this issue to me and any resource too in case it can help me solve this faster.