Implement Snap Find functionality

[CasparNuel]

Implement the necessary logic to process snap find / snap search commands.

An example of a standard snap find query:

http://localhost:8080/v2/snaps/find?
    architecture=amd64&
    confinement=strict%2Cclassic&
    fields=base%2Cconfinement%2Clinks%2Ccontact%2Cdescription%2Cdownload%2Clicense%2Cprices%2Cprivate%2Cpublisher%2Crevision%2Csummary%2Ctitle%2Ctype%2Cversion%2Cwebsite%2Cstore-url%2Cmedia%2Ccommon-ids%2Ccategories%2Cchannel&
    q=search-term