Additional source plugin filtering

I have added a possible implementation on a new merge request.
From the description there:

The underlying used library receives and passes the request query options to the List items request on the API.

In the same spirit, after exposing a way to pass directly those options in the migrate source plugin configuration, filtering is expanded to support any of the request query options in the API.

New commits on 3420528-add-source-filters topic branch and related new MR !14

• 102e629 Allow GC client list items request query options override
• 362f056 Declare new schema for migrate_plus.source.gathercontent_migration configuration
• d76227d Directly use migrate source plugin configuration
• b6de622 Include list items request query parameters on configuration schema

Posted minor feedback on MR.

@heddn, I have added types and labels to the new client_options key.

Please notice that even if API documentation uses types like boolean, or array of type; at the end they are represented as a comma separated values string.
Hence, I am reflecting that instead, given the added option tries to reflect directly what will be passed to the underlying library, and ultimately to the API.

New commits added to 3420528-add-source-filters topic branch and related MR !14

- 53dcd97 Add types and labels to client_options mapping on source schema

It works. Really well. LGTM.