feat: interoperable Cloud Assembly contract

[mrgrain]

Issue

Relates to aws/aws-cdk#32998

Reason for this change

We are publishing the cx-api package twice: Once as a standalone package @aws-cdk/cx-api and once as part of the construct library under aws-cdk-lib/cx-api. The code is copied during the release and the same versions of the packages will have the same code.

However this makes it difficult for other packages to take a type dependency on types from this package. The most common class that's used from cx-api is CloudAssembly - the result of app.synth(). Previously a package had to take a dependency on the very large aws-cdk-lib just to use a single type. It would be better if other packages could instead depend on the smaller, much more focused @aws-cdk/cx-api package.

Description of changes

In aws/aws-cdk#32998 we are adding a mechanism to detect cross-library compatibility of the CloudAssembly class. However, that alone doesn't help us with type checking. Instead we introduce a new type ICloudAssembly into the Cloud Assembly Schema. This interface only declares a single property: directory. Consumers can use this type to indicate where they would like to receive a CloudAssembly. They can then use runtime code to either confirm a provided object already satisfies the requirements or fallback to creating a new CloudAssembly from the directory.