find command should support skip argument

[b1ron]

What should be done?

We should support skip parameter for find and count commands. Do that only for the PostgreSQL handler.

Compat test cases should be extended to include skipDocs int field and use it. (We will rename the existing skip string that is used for skipping tests later).

We don't push down sorting, so that issue should be straightforward.

Where?

https://github.com/FerretDB/FerretDB/blob/main/internal/handlers/pg/msg_find.go
https://github.com/FerretDB/FerretDB/blob/main/internal/handlers/pg/msg_count.go

Definition of Done

• PostgreSQL handler updated;
• compatibility tests updated;
• spot refactorings done;
• user documentation updated.

test cases

• negative numbers
• non integers
• etc