Add dedicated schema parameters to tune auth mounts

[vinay-gopalan]

Description

Tuning auth mounts is currently managed by a resource block, which:

• manages durations using strings and
• relies on sys/auth/<path>/tune as the source of truth instead of sys/mounts/auth/<path> (there is a mismatch is the data returned by the two endpoints)

Both of these above make it hard to fix tune block configurations and states without potentially breaking users/causing drifts, hence this PR adds dedicated schema params for the fields previously managed by the tune block, and adds methods to easily manage them and add/tweak things in the future.

Relates OR Closes #2234

Checklist

• Added CHANGELOG entry (only for user-facing changes)
• Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestJWTAuthBackend_authMountSchema'
=== RUN   TestJWTAuthBackend_authMountSchema
=== PAUSE TestJWTAuthBackend_authMountSchema
=== CONT  TestJWTAuthBackend_authMountSchema
--- PASS: TestJWTAuthBackend_authMountSchema (1.57s)
PASS