Migrate from http/1.1 and websockets to http/2

[holgerkoser]

What this PR does / why we need it:
With this PR we will replace the http/1.1 got library and the websocket ws library with the native http/2 implementation.

We expect the following advantages:

1. Communication to the garden / seed api-servers should be much faster:

   1. reduced latency
   2. full request and response multiplexing
   3. compression of HTTP header fields
   4. tls session resumption

2. Simpler, and more robust implementation

   1. a single http2 session for all watches is kept between the list and the watch call (solves the problem of different api-server instances with inconsistent watch caches, "too old/new resourceVersion")
   2. Better aligned with the go-client implementation
   3. Introduced the Informer class used for both watches and cache synchronization

Which issue(s) this PR fixes:
Fixes #656

Special notes for your reviewer:

Release note:

The outgoing communication to all apiservers is done via `http/2` for read and write operations as well as for watches. It is assumed that these apiservers are accessible via `http/2`. This has the following advantages:
- Better performance due to reduced latency, full request and response multiplexing, HTTP header field compression and tls session resumption.
- Simpler, and more robust implementation aligned with the go-client implementation. 
- A single http2 session for all watches is kept between the list and the watch call which solves the problem with diverged watch-caches on different apiserver instances.

[grolu]

/lgtm