Add feature flag for OIDC workload identity credential types (#16348)

Add install-time feature flag for OIDC workload identity credential types

Implements FEATURE_OIDC_WORKLOAD_IDENTITY_ENABLED feature flag to gate
HashiCorp Vault OIDC credential types as a Technology Preview feature.

When the feature flag is disabled (default), OIDC credential types are
not loaded into the plugin registry at application startup and do not
exist in the database.

When enabled, OIDC credential types are loaded normally and function
as expected.

Changes:
- Add FEATURE_OIDC_WORKLOAD_IDENTITY_ENABLED setting (defaults to False)
- Add OIDC_CREDENTIAL_TYPE_NAMESPACES constant for maintainability
- Modify load_credentials() to skip OIDC types when flag is disabled
- Add test coverage (2 test cases)

This is an install-time flag that requires application restart to take
effect. The flag is checked during application startup when credential
types are loaded from plugins.

Fixes: AAP-64510

Assisted-By: Claude Sonnet 4.5 <noreply@anthropic.com>

awx/main/constants.py

@@ -11,6 +11,7 @@ __all__ = [
     'CAN_CANCEL',
     'ACTIVE_STATES',
     'STANDARD_INVENTORY_UPDATE_ENV',
+    'OIDC_CREDENTIAL_TYPE_NAMESPACES',
 ]
 
 PRIVILEGE_ESCALATION_METHODS = [
@@ -140,3 +141,6 @@ org_role_to_permission = {
     'execution_environment_admin_role': 'add_executionenvironment',
     'auditor_role': 'view_project',  # TODO: also doesnt really work
 }
+
+# OIDC credential type namespaces for feature flag filtering
+OIDC_CREDENTIAL_TYPE_NAMESPACES = ['hashivault-kv-oidc', 'hashivault-ssh-oidc']