gcs: add support for several system metadata keys

[antiacus]

What is the purpose of this change?

The branch adds support for six system GCS metadata keys in metadata filters (refer to https://rclone.org/filtering/#metadata):

• cache-control (Specifies directives for caches along the request/response chain.)
• content-disposition (Suggests a default filename for downloaded content or indicates how the content should be displayed.)
• content-encoding (Informs the recipient how to decode the content to obtain the original data.)
• content-language (Indicate the language of the textual content in the response.)
• content-type (Indicates the media type or MIME type of the resource in the HTTP message's body.)
• tier (Storage class of the object.)

The implementation is very similar to the one for the S3 backend.

Was the change discussed in an issue or in the forum before?

No, I haven't seen such disucussions at least.

Checklist

• I have read the contribution guidelines.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• I'm done, this Pull Request is ready for review :-)