Support "op" in the "columnFilters" entries

When you do a search in the UI, the result is that a structure like this is sent to mod-ldp (edited for brevity):

Each entry in the columnFilters array should also support an optional "op" key, whose value is one of the SQL relational operators =, <, etc. If absent, the it should default to = (as is always the case now).

Minor modifications will also be needed to the machine-readable documentation:

• Query schema at https://github.com/folio-org/mod-ldp/blob/master/ramls/query-schema.json

• Example query at https://github.com/folio-org/mod-ldp/blob/master/ramls/examples/query-example.json

And no doubt more tests will be required – but hopefully trivial.