[google-auth] Feat: ECP and hardware-backed cert support for X.509 workloads

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this feature request.

### Summary of the feature request

I would like to use my hardware-backed (TPM, Secure Enclave, HSM, etc) certificates with a X.509 Workload Identity Provider.

Today google-auth X.509 provider assumes it will be given the private key of the certificate and hard fails if it doesn't:

[packages/google-auth/google/auth/external_account.py](https://github.com/googleapis/google-cloud-python/blob/main/packages/google-auth/google/auth/external_account.py#L480-L484) line 480:
```python
# Inject client certificate into request.
if self._mtls_required():
request = functools.partial(
request, cert=self._get_mtls_cert_and_key_paths()
)
```

this limits the implementation to only private keys stored directly on the filesystem.

Allowing hardware-backed certificates provides iron-clad security for service account authentication. The library will be able to authenticate as a WIF Identity / Service account using a private key that was generated on the TPM / Secure Enclave and cannot be exported.

### Desired code experience

```python
from google.auth import default
from google.auth.transport.requests import AuthorizedSession

# certificate_config.json has ECP libs + macos_keychain (hardware-backed key)
# but NO "workload" section (no private key file on disk)

creds, _ = default(scopes=["https://www.googleapis.com/auth/cloud-platform"])
session = AuthorizedSession(creds)
session.configure_mtls_channel() # ECP handles mTLS via hardware signing
response = session.get("https://cloudresourcemanager.googleapis.com/v1/projects/my-project")
```

### Expected results

`_get_cert_bytes()` should fall back to the [ECP signer library](https://github.com/googleapis/enterprise-certificate-proxy) `(GetCertPemForPython)` to retrieve the certificate from the OS keystore when no workload file path exists. `_perform_refresh_token()` should skip injecting `cert=(cert_path, key_path)` when paths are `(None, None)`, letting the session's ECP adapter (via `configure_mtls_channel())` handle mTLS. Currently it crashes with `TypeError` (None path) or `SSLError: [SSL] PEM lib`s (placeholder path).

### API client name and version

google-auth 2.56.2

### Use case

This feature would be useful in allowing non-GCP service account / WIF authenticating while never needing to expose a private key for export and theft.

### Additional context

I'm glad to discuss further internally next week. go/teams/jayhlee.