YT-21156: Implement THttpChannel and tests for http RPC

* `THttpChannel` supports both http and https.
* `TRpcOverHttpImpl` in tests creates a THttpChannel and http or https server.

Fixes:
* When `TFDConnection` is shut down, also call OnPeerDisconnected_ handlers. When combined with a poller owned by a http server, this allows to cancel all requests when the server is stopped.
* In `TTlsConnection`, use MakeWeak instead of MakeStrong for background reading, so that the channel is actually destroyed when there are no external references (e.g. http request is cancelled).
* In `TTwoLevelFairShareQueue`, prevent a deadlock when `Drain()` is called from within Drain() when an action is cleared.
* In `AcceptSocket`, treat `EMFILE` (Out of file descriptors) as a transient error rather than crashing a server.
* In http rpc server, add new headers support: `X-YT-User`, `X-YT-Rpc-Protocol-Version-Major`/`Minor`; put unknown `X-` headers into `custom_metadata_ext`.
* Expose known http header name constants in http/helpers.h (in their own sub-namespace).
b2d1d5224f6c5bcd9346140ef580f6b65a96361d

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