Custom metadata on bucket objects with the plugin-cloud-storage

[immotus]

The current implementation of @payloadcms/plugin-cloud-storage doesn't allow for users to configure custom metadata to be set on newly uploaded objects. The fix should be straightforward, as all functionality is pretty much there. The only thing that needs to be done is to allow users to specify custom metadata in their collection configuration settings.

Reason:
For example, the default caching policy on GCP public objects is Cache-Control: max-age=3600, whereas for static objects, I'd rather set something like Cache-Control: public, max-age=15552000, must-revalidate.

For example:

The https://github.com/payloadcms/payload/blob/main/packages/plugin-cloud-storage/src/adapters/gcs/handleUpload.ts already set file metadata:

    await gcsFile.save(file.buffer, {
      metadata: {
        contentType: file.mimeType,
      },
    })

It would be great to allow the passing of custom metadata objects, e.g.:

    await gcsFile.save(file.buffer, {
      metadata: {
        contentType: file.mimeType,
        ...customFileMetadata
      },
    })

It just needs to be integrated into the common API pattern to allow custom data to be passed on, like implemented in many other places.