(api-gateway): Support SpecRestApi for UsagePlan

### Describe the feature

Currently, when you use a `UsagePlan` L2 construct, you must specify a `Method` L2 construct to assign to the throttling-per-method configuration. Unfortunately, if you use a `SpecRestApi` with a Swagger/OpenAPI definition, there is no L2 Method construct to link to. It would be very helpful if you could initialize a skeleton `Method` instance for this purpose, or even better, have better processing of the Swagger template so that you could add the usage plan on a method after initialization.

### Use Case

Working with Swagger/OpenAPI and needing to initialize a `UsagePlan` for the API.

### Proposed Solution

Something like:

```ts
const api = new SpecRestApi(this, 'API', {
apiDefinition: ApiDefinition.fromInline('...'),
});

const usagePlan = this.api.addUsagePlan('UsagePlan', {...});
usagePlan.addApiStage({
api,
throttle: [Method.fromSwagger('my-method')],
});
```

or

```ts
const api = new SpecRestApi(this, 'API', {
apiDefinition: ApiDefinition.fromInline('...'),
});

// Precondition: we know this API returns special "Swagger" UsagePlan instance
const usagePlan = this.api.addUsagePlan('UsagePlan', {...});
usagePlan.addRule({
api,
method: 'my-method', // this then searches and validates the existing config
});
```

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.42.0

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

macOS 12.6

Start by tracing the SpecRestApi, UsagePlan, and ApiStage APIs involved in linking throttling rules to methods, then inspect how Swagger/OpenAPI definitions are represented. Decide whether the supported behavior should provide a method reference or validate a method name from the definition; done means a SpecRestApi can configure per-method usage-plan throttling with validation and coverage for the chosen API.

Written by the indexing model from the issue text.