software.amazon.awscdk.services.apigateway.SpecRestApi: adjust method to add integration

### Describe the feature

I would like to create an API by an openapi schema. But i want to change one method of the API to add an integration to it. I don't want to change the openapi schema (using AWS tags to configure the integration!).

Currently this is impossible to do? right?

### Use Case

I want to add the integration on POST method in CDK itself.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

latest

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

linux

Start with the SpecRestApi entry point and inspect how methods and integrations are created from an OpenAPI schema. Reproduce the POST-method use case described in the issue, then determine the API and test coverage needed to add an integration without changing the schema.

Written by the indexing model from the issue text.