Add support for sync resource in HCP Vault Secrets

[rodrigo-hcp]

Test scenarios

• Import an app with one or more syncs that are unknown to the local TF state and then setting an an empty sync_names: plan will warn of removal of syncs from app.
• Validates that a config block must be present in the sync resource
• Importing a sync works
• Adding or removing one or more syncs from app works

🛠️ Description

🏗️ Acceptance tests

• Are there any feature flags that are required to use this functionality?
• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch?

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccVaultSecretsResourceSync'

...

[pmmukh]

would rec a 2nd review, not well versed in TFP thing, but overall looks great to me!