Scoped Webauthn Challenges

[Joerger]

Implements scoped Webauthn challenges as described in RFD 155.

Follow up PRs:

• e PR
• Add audit events and product metrics
• Prompt for reusable MFA when needed to avoid redundant mfa prompts (tctl users add, tctl create muliple-resources.yaml, etc.`)

[Joerger]

Sorry @rosstimothy and other reviewers, I ran into some issues during implementation of the reuse logic and need to revert/refactor some of this. I'm going to include the reuse implementation in this PR and reopen it once it's ready. It should be a simpler change overall after this.

[Joerger]

@rosstimothy @codingllama @gabrielcorado this is ready for review when you get a chance

[Joerger]

Did some more testing and found some issues. Propagating the scope and AllowReuse through the full client->server->client->server MFA flow is problematic for backwards compatibility and differences in the Web MFA flow.

I have some MFA refactors under way that should significantly reduce the complexity of this part of the change. I will close this and open a new series of PRs once it's ready.