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 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.

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