Add support for write-only attributes

[SarahFrench]

Closes TF-22967

This PR:

• adds support for the new write-only schema behaviour.
  • The new write-only data was added to the terraform providers schema -json command in TF-18620: add write-only attributes to protobuf definitions terraform#35989
• refactors some pre-existing tests when adding new tests

I've not included any changes in the sanitize package because that package appears to be specific to Sensitive values, and is irrelevant to ephemeral resources & write-only attributes.

[SarahFrench]

Merging to main now, even though the WO attribute feature is still in development. Releasing these changes prior to that feature being released should be fine, the same way that fields currently tolerate having some schema behaviours as false. I'll seek some feedback before making a release though.