NetworkBridge: validator (authorities) discovery api (#1699)

* stupid, but it compiles

* redo

* cleanup

* add ValidatorDiscovery to msgs

* sketch network bridge code

* ConnectToAuthorities instead of validators

* more stuff

* cleanup

* more stuff

* complete ConnectToAuthoritiesState

* Update node/network/bridge/src/lib.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Collator protocol subsystem (#1659)

* WIP

* The initial implementation of the collator side.

* Improve comments

* Multiple collation requests

* Add more tests and comments to validator side

* Add comments, remove dead code

* Apply suggestions from code review

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Fix build after suggested changes

* Also connect to the next validator group

* Remove a Future impl and move TimeoutExt to util

* Minor nits

* Fix build

* Change FetchCollations back to FetchCollation

* Try this

* Final fixes

* Fix build

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* handle multiple in-flight connection requests

* handle cancelled requests

* Update node/core/runtime-api/src/lib.rs

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* redo it again

* more stuff

* redo it again

* update comments

* workaround Future is not Send

* fix trailing spaces

* clarify comments

* bridge: fix compilation in tests

* update more comments

* small fixes

* port collator protocol to new validator discovery api

* collator tests compile

* collator tests pass

* do not revoke a request when the stream receiver is closed

* make revoking opt-in

* fix is_fulfilled

* handle request revokation in collator

* tests

* wait for validator connections asyncronously

* fix compilation

* relabel my todos

* apply Fedor's patch

* resolve reconnection TODO

* resolve revoking TODO

* resolve channel capacity TODO

* resolve peer cloning TODO

* resolve peer disconnected TODO

* resolve PeerSet TODO

* wip tests

* more tests

* resolve Arc TODO

* rename pending to non_revoked

* one more test

* extract utility function into util crate

* fix compilation in tests

* Apply suggestions from code review

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>

* revert pin_project removal

* fix while let loop

* Revert "revert pin_project removal"

This reverts commit ae7f529d8de982ef66c3007dd1ff74c6ddce80d2.

* fix compilation

* Update node/subsystem/src/messages.rs

* docs on pub items

* guide updates

* remove a TODO

* small guide update

* fix a typo

* link to the issue

* validator discovery: on_request docs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>
Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>
Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>