Add API Keys spec (create, list, revoke)

[cwperks]

Description

Adds OpenAPI specification for the API Keys feature in the OpenSearch Security plugin.

Related to opensearch-project/security#5443

Operations added:

• security.create_api_key — POST /_plugins/_security/api/apitokens
• security.get_api_keys — GET /_plugins/_security/api/apitokens
• security.delete_api_key — DELETE /_plugins/_security/api/apitokens/{id}

Schemas added:

• ApiKey, ApiKeyIndexPermission, ApiKeyCreateRequest, ApiKeyCreateResponse, ApiKeyListResponse, ApiKeyRevokeResponse

Test story: tests/plugins/security/api/apitokens.yaml

All operations are marked x-version-added: '3.7' and excluded from amazon-managed and amazon-serverless distributions.

Issues Resolved

Related to opensearch-project/security#4009

Check List

• Spec lint passes
• Spec merger succeeds
• Test story included
• Commits are signed per the DCO using --signoff

[Tokesh]

Hi @cwperks ! Thanks for your contribution.
it seems like the CI is failing. Could you please take a look at the logs? Let me know if you need any help getting it green!