Add new resource SPIFFE config

[stevendpclark]

Description

Initial work to add a new resource for the SPIFFE auth engine's configuration endpoint.

Docs still missing.

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=TestAccXXX'
=== RUN   TestAccSpiffeAuthConfig
--- PASS: TestAccSpiffeAuthConfig (2.51s)
=== RUN   TestAccSpiffeAuthRole
--- PASS: TestAccSpiffeAuthRole (2.11s)
PASS
...

Community Note

• Please vote on this pull request by adding a 👍 reaction to the original pull request comment to help the community and maintainers prioritize this request
• Please do not leave "+1" comments, they generate extra noise for pull request followers and do not help prioritize the request

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.