I've completely redesigned `RestClient` component ...

# kvision

[Robert Jaros]

I've completely redesigned RestClient component and put it in the new kvision-rest module for KVision 5. It's now based on functional config builders (a bit similar to Ktor client) instead of method parameters. To make migration easier I've moved the whole old jQuery based client to kvision-jquery module and deprecated it for now. See kvision_5 branch for more details.