NCBC-3318: Expose KV Options public, refactored slightly

Motivation:
Clean up previous commit a little, eliminating the increase in top-level public types.

Modifications:
* Moved all "ReadOnly" versions to member classes inside the existing *Options classes.

Change-Id: I0edc73b17cd01fe4019edaac634feb29fd6d0c26
Reviewed-on: https://review.couchbase.org/c/couchbase-net-client/+/185174
Tested-by: Build Bot <[email protected]>
Reviewed-by: Jeffry Morris <[email protected]>
Reviewed-by: Emilien Bevierre <[email protected]>