Add swagger pattern for filtering param

[Martoon-00]

For swagger of sorting params, we fill pattern field, and this allows for validation of user input when trying executing requests from Swagger UI.

Would be sweet to have a similar thing for filtering. This should be slightly more difficult, but still doable.

A good start would be to run stack ghci servant-util:servant-util-test, execute printFilterSwagger, put the resulting file to Swagger Editor, and make the descriptions of fields match with what is actually accepted in that field when "Try it out" and then "Execute" are pressed.