(s3-deployment): BucketDeployment expose asset hash

### Describe the feature

I would like to be able to access some kind of hash for the assets in a BucketDeployment.

### Use Case

I have an s3 bucket with some sql schema migrations, and I want to run a Custom Resource Provider when the lambda or the migration files change.

### Proposed Solution

Add hashes to the [array](https://github.com/aws/aws-cdk/blob/a7de7feb6a14658ec25f4cfda434d5e1d69157d2/packages/aws-cdk-lib/aws-s3-deployment/lib/bucket-deployment.ts#L279C29-L279C41) of [`SourceConfigs`](https://github.com/aws/aws-cdk/blob/0d6ef63e04ed0301e40684594266e83b50dce186/packages/aws-cdk-lib/aws-s3-deployment/lib/source.ts#L13) in a `BucketDeployment`, and add a getter to hash all of them.

### Other Information

This *can* be done if there is only one source with
```
const migrationFilesHash: string = (migrationFilesDeployment.node.findChild("Asset1")).assetHash;
```
but I don't like relying on the magic string `Asset` because that isn't part of any api agreement.

### Acknowledgements

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

### CDK version used

2.130.0

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

debian stable

Start with packages/aws-cdk-lib/aws-s3-deployment/lib/bucket-deployment.ts at the SourceConfigs array and packages/aws-cdk-lib/aws-s3-deployment/lib/source.ts at the SourceConfigs definition. Trace how deployment assets are represented, then expose their hashes through the BucketDeployment API and provide a combined hash getter as described; done means consumers no longer need the magic Asset1 child name.

Written by the indexing model from the issue text.