Support for stable m.oauth UIA stage for MSC4312

[hughns]

This PR adds support for the stable m.oauth UIA stage used for resetting the cross-signing identity when using the OAuth 2.0 client authentication API. This was proposed in MSC4312.

It maintains support for unstable org.matrix.cross_signing_reset UIA stage to allow time for clients to adopt the stable version. This is achieved by offering two UIA flows (one for the unstable, and another for the stable) with a single stage in each.

I've added some test coverage of the expected UIA response from POST /_matrix/client/v3/keys/device_signing/upload too.

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[anoadragon453]

A couple minor changes.

org.matrix.cross_signing_reset being the unstable stage name, yet the stable action name is quite confusing!

[hughns]

org.matrix.cross_signing_reset being the unstable stage name, yet the stable action name is quite confusing!

Indeed. Thanks for adding the clarification comments on this. 👍

[anoadragon453]

LGTM. Thanks!