servicecatalogappregistry: lookupFromName for application

### Describe the feature

The ability to lookup a Service Catalog AppRegistry Application via it's Name or Id.

### Use Case

Once https://github.com/aws/aws-cdk/pull/20850 lands, it'll be possible to create a central "application" and then share it. This will enable creating multi-env deployments and associating them back to a single application. A possible solution to make this work is to bootstrap the SCAR App and then store the Arn in context, but to make it more "automatic" it'd be nice to define a name and then look it up.

### Proposed Solution

AWS SC AppRegistry exposes an API to get applications by name: https://docs.aws.amazon.com/servicecatalog/latest/dg/API_app-registry_GetApplication.html.

Expose this as `Application.fromLookupName()` which returns a hydrated application that can then be used to `associateStack`.

For efficiencies sake, it might make sense to have an opt in behavior (either via prop or flag) to have the context key _not_ be account/env specific for the likely to be more common case that there's a single application which is then shared across org (and therefore is technically unique).

### Other Information

An alternative would be a way to write out the resulting ARN from creation directly to context, but I'm not sure that's possible and/or desired? If that's possible though, it could obviate the need for this.

Related, there was a neat idea proposed [here](https://github.com/aws/aws-cdk-cli/issues/787) which had the potential idea of using AppRegistry as an integrated primitive. This FR could potentially underpin that behavior for multi-stack applications.

### Acknowledgements

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

### CDK version used

2.30.0

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

N/A

Start at the servicecatalogappregistry Application entry point and compare the requested Application.fromLookupName API with AWS Service Catalog AppRegistry's GetApplication operation. Verify that the lookup returns a hydrated application usable by associateStack, and resolve whether context should be account/environment-specific or support an opt-in shared key before considering the feature complete.

Written by the indexing model from the issue text.