Ability to require reason for access request

[programmerq]

What would you like Teleport to do?

Add the ability to set a policy to make a reason required for an access request to be submitted.

What problem does this solve?

Improves user experience for environments with audit/compliance policies that require a reason. Currently, an explicit denial process is required to ensure this requirement. Adding the ability to mark a reason as required would allow both tsh and the web UI to prompt for a reason without the end user needing to remember that it's required.

This is separate from the forced access request feature that can prompt for a reason. This is meant for a team that only occasionally needs access to privileged resources, and they don't need to do access requests for their day-to-day work.

If a workaround exists, please include it.