New blob_properties transport parameter for GCS

[FHTheron]

Title

Add GCS transport parameter blob_properties to allow custom properties to be specified for newly created blobs.

Motivation

This allows setting the GCS file (or blob) content_type and content_encoding (or any other GCS blob properties) regardless of the filename. Callers can use mimetypes.guess_type or manually specify specific values.

Example:

smart_open.open("gs://somewhere/somefile.json.gz", transport_params={
  "blob_properties": {
    "content_type": "application/json",
    "content_encoding": "gzip"
  }
}

Possibly useful for #429

Checklist

• Picked a concise, informative and complete title
• Clearly explained the motivation behind the PR
• Linked to any existing issues that your PR will be solving
• Included tests for any new functionality
• Checked that all unit tests pass