OData support for $select

A question came in from Stack Overflow on OData with Linq2Couchbase: https://stackoverflow.com/questions/61078681/how-to-apply-odata-select-after-couchbase-iqueryable-is-executed

A minimum source code to reproduce the issue is here: https://github.com/ofthelit/ODataCouchbase

It looks like `$select` causes a `NotImplementedException`. I ran the code, and found that the N1QL itself is generated correctly and executes correctly. But in BucketQueryExecutor.cs, ExecuteCollection, running the query results in the exception. Supporting a `$select` seems like a common scenario in OData. Is this a bug, or is this just not yet supported? What would it take to get it supported?

(See also issue #222)