apigatewayv2: Custom domain configuration for WebSocket API

### Describe the feature

HTTP API construct has [defaultDomainMapping](https://docs.aws.amazon.com/cdk/api/v2/docs/@aws-cdk_aws-apigatewayv2-alpha.HttpApi.html#defaultdomainmappingspan-classapi-icon-api-icon-experimental-titlethis-api-element-is-experimental-it-may-change-without-noticespan) property to set a custom domain for the API. We'd like to have an equivalent property for WebSocket API.

### Use Case

Set a custom domain for WebSocket API.

### Proposed Solution

We can add `defaultDomainMapping` option and create `CfnMapping` resouce like the below:

```ts
new CfnApiMapping(this, 'Mapping', {
apiId: api.apiId,
domainName: domainName.name,
stage: stage.stageName,
});
```

We may additionally need to create a `CfnStage` because currently it is not created inside `WebSocketApi`. In HTTP API we have [createDefaultStage](https://docs.aws.amazon.com/cdk/api/v2/docs/@aws-cdk_aws-apigatewayv2-alpha.HttpApi.html#createdefaultstagespan-classapi-icon-api-icon-experimental-titlethis-api-element-is-experimental-it-may-change-without-noticespan) option for this purpose.

### Other Information
Currently we can set a custom domain by the code like below:

```ts
const api = new WebSocketApi(this, 'Api', {
connectRouteOptions: {
authorizer,
integration: new WebSocketLambdaIntegration('ConnectIntegration', websocketHandler),
},
disconnectRouteOptions: {
integration: new WebSocketLambdaIntegration('DisconnectIntegration', websocketHandler),
},
defaultRouteOptions: {
integration: new WebSocketLambdaIntegration('DefaultIntegration', websocketHandler),
},
});

const stage = new WebSocketStage(this, `Stage`, {
webSocketApi: api,
stageName: this.defaultStageName,
autoDeploy: true,
});

const dnsRecordName = 'foo';
const customDomainName = `${dnsRecordName}.${props.hostedZone.zoneName}`;

const domainName = new DomainName(this, 'DomainName', {
domainName: customDomainName,
certificate: new Certificate(this, 'Certificate', {
domainName: customDomainName,
validation: CertificateValidation.fromDns(props.hostedZone),
}),
});

new CfnApiMapping(this, 'Mapping', {
apiId: api.apiId,
domainName: domainName.name,
stage: stage.stageName,
});

new ARecord(this, 'DnsRecord', {
recordName: dnsRecordName,
zone: props.hostedZone,
target: RecordTarget.fromAlias(
new ApiGatewayv2DomainProperties(domainName.regionalDomainName, domainName.regionalHostedZoneId),
),
});
```

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.61.1

### Environment details (OS name and version, etc.)

macOS

Start by comparing HttpApi's defaultDomainMapping and createDefaultStage behavior with the WebSocketApi and WebSocketStage constructs. Review the proposed CfnApiMapping and possible CfnStage resources, then define completion as equivalent custom-domain mapping support for WebSocket APIs, including the required default stage behavior.

Written by the indexing model from the issue text.