Implements Auth provider configuration management APIs (#415)

* Implements Auth provider configuration management APIs for configuring SAML and OIDC providers.

```
listProviderConfigs(
  options: admin.auth.AuthProviderConfigFilter
): Promise<admin.auth.ListProviderConfigResults>;
getProviderConfig(providerId: string): Promise<admin.auth.AuthProviderConfig>
deleteProviderConfig(providerId: string): Promise<void>;
updateProviderConfig(
  providerId: string, updatedConfig: admin.auth.UpdateAuthProviderRequest
): Promise<admin.auth.AuthProviderConfig>;
createProviderConfig(
  config: admin.auth.AuthProviderConfig
): Promise<admin.auth.AuthProviderConfig>;
```

Author: bojeil-google

CHANGELOG.md

@@ -1,6 +1,10 @@
 # Unreleased
 
--
+- [feature] Added the provider config management APIs for managing OIDC and SAML
+  provider configurations (CRUD) via
+  `auth.listProviderConfigs()`, `auth.getProviderConfig()`,
+  `auth.deleteProviderConfig()`, `auth.updateProviderConfig()` and
+  `auth.createProviderConfig()`.
 
 # v7.2.0