Add new WIF fields for GCP Auth (Vault Enterprise only)

[vinay-gopalan]

Description

Adds the following new fields to the GCP Auth Backend resource to enable the WIF workflow:

• identity_token_audience
• identity_token_ttl
• identity_token_key
• service_account_email

Checklist

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

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestGCPAuthBackend_WIF'
=== RUN   TestGCPAuthBackend_WIF
--- PASS: TestGCPAuthBackend_WIF (1.96s)
PASS

[Zlaticanin]

Should be add another usage example here for WIF? Similar how we've done for others

[vinay-gopalan]

Should be add another usage example here for WIF? Similar how we've done for others

Ah thanks for the reminder! Added examples for both Auth and Secrets