feat: Documenting custom credential suppliers

[vverman]

Adding documentation for Custom Credential Suppliers.

Custom Credential Suppliers enable developers to securely integrate third-party authentication directly into the Google Cloud SDKs. Custom Credential Suppliers are primarily used to handle authentication in non-standard cloud environments.

The design and scopes for this are documented

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• [x] I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• [x] Tests pass: npm test (see Testing)
• [x] Lint pass: npm run lint (see Style)
• [x] Required CI tests pass (see CI testing)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [x] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [x] Please merge this PR for me once it is approved

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

These tests will safely skip if the env variables aren't provided.

For the auth/system-test/customCredentialSupplierAws.test.js, we need: AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION, GCP_WORKLOAD_AUDIENCE, GCS_BUCKET_NAME. Please refer to the ‎auth/README.md under custom credential suppliers for AWS.

For the auth/system-test/customCredentialSupplierOkta.test.js, we need: OKTA_DOMAIN, OKTA_CLIENT_ID, OKTA_CLIENT_SECRET, GCP_WORKLOAD_AUDIENCE, GCS_BUCKET_NAME. Please refer to the auth/README.md under custom credential suppliers for Okta.