Add "cursor" query parameter

[avtobiff]

• Add draft text for "cursor" query parameter.

• Add examples for "cursor" query parameter.

• Add ietf-list-pagination:cursor-not-found identity.

• Add cursor-param-grouping grouping.

[avtobiff]

I did not put any motivation for "cursor" query parameter in the updated draft text.

I.e. nothing is mentioned about it not locking the database and just forming a window, that this window can be more stable than limit+offset pagination, can be more efficient then limit+offset (due to using underlying db index).

Should this mentioned?