Moved all okhttp code to dedicated okhttp package in order to prepare…

[patrickunterwegs]

… library for further implementations like ktor

[rfc2822]

@ArnyminerZ @sunkup Our plan is to

1. Move existing code into a okhttp package so that we can continue to use it until the Ktor version is ready.
2. The Ktor code will be placed in a separate ktor package.
3. As soon as it's ready, we can use the ktor package in DAVx5, ideally first only for WebDAV.
4. If this is working, we can migrate the rest of DAVx5 to the Ktor package.
5. Then the okhttp package can be removed in dav4jvm.

For DAVx5, we will keep okhttp as engine for Ktor, but it shouldn't be a hard dependency, so that other projects can use dav4jvm with other engines.