Add a new `skip` counter in diff.summary()

[dgarros]

Environment

• DiffSync version: 1.3.0

Proposed Functionality

Extend the list of counter returned by diff.summary() to include skip in addition to create, update, delete & no-change

Use Case

Currently the models that are being skipped because of some global or model flags like SKIP_UNMATCHED_SRC | SKIP_UNMATCHED_DST are not accounted for in the diff