Add metadata to signed artifacts when using `sam deploy --signing-profiles ... --metadata ...`

### Describe your idea/feature/enhancement

I would like the `sam deploy` and `sam package` commands to apply metadata when using the `--metadata` flag when I am signing artifacts using AWS signer.

Currently, the `--metadata` values are only written to unsigned artifacts. When the signer jobs finish and write the signed artifacts to the given bucket, they do not include the metadata.

So the enhancement would be to include this metadata on the signed artifacts.

### Proposal

Shouldn't require any changes to the SAM spec, just to the CLI.

The unsigned artifact locations should be already tracked by the CLI and the signed artifact locations could be determined from the AWS signer responses. Using these two locations, the metadata could be copied across when the signing completes and before any other operations are done.