Make the IS optional during discovery

[jryans]

This would implement matrix-org/matrix-spec-proposals#2284.

At the moment, if the HS .well-known includes an IS but it is invalid, this is a hard error. Following the above MSC would allow the user to continue.

[jryans]

This is blocked on the MSC progressing. We aren't able to use the unstable feature process for .well-known changes.

[jryans]

FCP has now begun on the MSC. It should be safe to merge this once FCP has ended.

[jryans]

It's now safe to proceed here.