Use Ktor for HTTP #1804
Description
Since dav4jvm is being rewritten to Ktor (bitfireAT/dav4jvm#72) mainly for multiplatform compatibility, we want to rewrite DAVx5 to Ktor too, mostly to make use of suspending HTTP calls (which will allow us to drop all the runInterrupting {} boilerplate code and come handy for coroutine-using code like the SyncManager).
It will then also be compatible with multiple platforms which is not really a thing at the moment, but who knows.
The plan is to slowly transition to Ktor in DAVx5 and still use okhttp as the client engine so that we can keep custom okhttp configuration.
Rewriting also includes changing HttpUrl into Ktor Url where possible.