Refactor `services.AccessMFAParams` and the accompanying `MFAParams` methods so
they may be extended to carry additional data (eg, information required to
perform device trust checks).
This PR refactors existing code without adding any new functionality, so it may
be backported to older branches to alleviate eventual conflicts. Follow up
changes will take advantage of it to add support for device trust.
Related to gravitational/teleport.e#514.
Replace role/authPref RequireSessionMFA (bool) with RequireMFAType
(string).
- Add new RequireMFAType constant values with custom boolean marshalling.
- Add RequireMFAType to role and auth preference and deprecate
RequireSessionMFA.
- Add session-mfa override login when hardware_key_touch is enforced.
- Add protobuf enum for RequireMFAType.
- Add support for proto enums in protoc-gen-crd and update Kubernetes
Operator manifests.