Allow setting no expiry on `teleport_provision_token` resource

[AClarkie]

The teleport_provision_token resource requires a metadata.expires field, however it's now possible to create tokens without an expiry, for example the below yaml can be used:

kind: token
version: v2
metadata:
  name: am-token
spec:
  roles: [Bot]

  join_method: iam

I've spoken with @zmb3 and @stevenGravy and they believe this is a missing feature potentially following the change here, more context can be found on the below Slack thread
https://goteleport.slack.com/archives/CEZH6UL64/p1685450022714919