feat(apigateway): add mode property for SpecRestApi (#34198)

### Issue # (if applicable)
N/A

### Reason for this change
`AWS::ApiGateway::RestApi` has a `mode` property, but the L2 construct for API Gateway (SpecRestApi class) does not currently expose this property.  
Therefore, I added the `mode` property to the `SpecRestApiProps` interface.



### Description of changes



- Introduced an optional `mode` property in `SpecRestApiProps`.
- Created an enum class `RestApiMode` to define the possible values for `mode`. This enum includes two values: `merge` and `overwrite`.
- Added unit and integration tests.
- Updated the README.md for aws-apigateway.

### Describe any new or updated permissions being added


No new IAM permissions are introduced.

### Description of how you validated changes


- `packages/aws-cdk-lib/aws-apigateway/test/restapi.test.ts`
- `packages/@aws-cdk-testing/framework-integ/test/aws-apigateway/test/integ.spec-restapi.ts`

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.*

Author: tttol