feat: interoperable Cloud Assembly contract (#133)

### 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.

lib/cloud-assembly/index.ts

@@ -2,3 +2,4 @@ export * from './schema';
 export * from './metadata-schema';
 export * from './artifact-schema';
 export * from './context-queries';
+export * from './interfaces';

lib/cloud-assembly/interfaces.ts

@@ -0,0 +1,31 @@
+// The interfaces in this file, mainly exist __here__ because this is a convenient place to put them.
+// The Assembly Schema package is already a jsii package and a dependency of `aws-cdk-lib`.
+// It is effectively the only place we can put shared interfaces to be used across the jsii ecosystem.
+//
+// Putting a shared interface in here should be a huge exception.
+// It needs to be justified by great benefits it provides to the ecosystems.
+// All interfaces should be as minimal as possible.
+
+/**
+ * Interoperable representation of a deployable cloud application.
+ *
+ * The external and interoperable contract for a Cloud Assembly is
+ * a directory containing a valid Cloud Assembly.
+ *
+ * Implementations should use the directory to load the Cloud Assembly from disk.
+ * It is recommended that implementations validate loaded manifest files using
+ * the provided functionality from this package.
+ * Within an implementation, it may be prudent to keep (parts of) the Cloud Assembly
+ * in memory during execution and use an implementation-specific contract.
+ * However when an implementation is providing an external contract,
+ * this interface should be used.
+ */
+export interface ICloudAssembly {
+  /**
+   * The directory of the cloud assembly.
+   *
+   * This directory will be used to read the Cloud Assembly from.
+   * Its contents (in particular `manifest.json`) must comply with the schema defined in this package.
+   */
+  readonly directory: string;
+}