CognitoIDP[UserPoolClient]: Avoid underlying provider validation failure

* In a current state the instantiation of `UserPoolClient` fails with
```
cannot run refresh: refresh failed: reading Amazon Cognito IDP (Identity Provider) User Pool Client (): InvalidParameter: 1 validation error(s) found.
- minimum field size of 1, DescribeUserPoolClientInput.ClientId.
```
unless we explicitly set the `external-name` annotation. See #752 for more debugging data

* This change will preset the `id` with `spec.forProvider.name` to avoid
the validation failure. The `id` will be populated eventually with the
dynamic value from the cloud provider

* I also tried to follow the doc at https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/cognito_user_pool_client#import
but it seems to be wrong in practice:

If we use smth like

```
TemplatedStringAsIdentifierWithNoName("{{ .parameters.user_pool_id }}/{{ .parameters.name }}"),
```

it will fail with
```
cannot run refresh: refresh failed: reading Amazon Cognito IDP (Identity Provider) User Pool Client (us-west-1_dMnICLKVB/): InvalidParameterException: 1 validation error detected: Value 'us-west-1_dMnICLKVB/' at 'clientId' failed to satisfy constraint: Member must satisfy regular expression pattern: [\w+]+
```

It proves that the import doc deviates from reality and fails its own
provider validation.

* It is possible that we observe the bug in the underlying terraform aws
provider.

* This fix provides best possible solution without the fix of underlying
provider and associated costs.

* Fixes #752

Signed-off-by: Yury Tsarev <yury@upbound.io>

config/externalname.go

@@ -111,7 +111,7 @@ var ExternalNameConfigs = map[string]config.ExternalName{
 	// us-west-2_abc123
 	"aws_cognito_user_pool": config.IdentifierFromProvider,
 	// us-west-2_abc123/3ho4ek12345678909nh3fmhpko
-	"aws_cognito_user_pool_client": config.IdentifierFromProvider,
+	"aws_cognito_user_pool_client": TemplatedStringAsIdentifierWithNoName("{{ .parameters.name }}"),
 	// auth.example.org
 	"aws_cognito_user_pool_domain": config.IdentifierFromProvider,
 	// us-west-2_ZCTarbt5C,12bu4fuk3mlgqa2rtrujgp6egq