@aws-cdk/aws-apigateway module: Add ability to reference execution log group ARN or Name

Adding the ability to reference the log group ARN or name that is generated by the `RestApi` construct.

### Use Case

To be able to do *anything* at all with the log group that gets generated (e.g. add a subscription filter) you need to be able to dynamically reference the name or ARN.

### Proposed Solution

Either:

* Create a readonly property on the instantiated `RestApi` class called `logGroupName` or `logGroupArn` that can be referenced by other constructs.
* Or, add the ability to set the execution log group name (like you can with access logs) so that you can have a consistent name to reference the log group by.

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start at the TypeScript RestApi construct and inspect how its generated execution log group is represented, alongside the existing access-log configuration. Done means another construct can reference the execution log group's ARN or name, or configure a stable name, with the chosen behavior fitting the issue's breaking-change concern.

Written by the indexing model from the issue text.