Add remote signer endpoint for GetOrchestratorInfo

[j0sh]

TODO explain

[codecov]

Codecov Report

❌ Patch coverage is 4.00000% with 96 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.46184%. Comparing base (aca2275) to head (f059422).

Files with missing lines	Patch %	Lines
server/remote_signer.go	0.00000%	73 Missing ⚠️
cmd/livepeer/starter/starter.go	16.00000%	21 Missing ⚠️
cmd/livepeer/starter/flags.go	0.00000%	2 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3791         +/-   ##
===================================================
- Coverage   31.52544%   31.46184%   -0.06360%     
===================================================
  Files            159         160          +1     
  Lines          47901       48001        +100     
===================================================
+ Hits           15101       15102          +1     
- Misses         31911       32009         +98     
- Partials         889         890          +1     

Files with missing lines	Coverage Δ
core/livepeernode.go	60.22099% <ø> (ø)
cmd/livepeer/starter/flags.go	0.00000% <0.00000%> (ø)
cmd/livepeer/starter/starter.go	21.90476% <16.00000%> (-0.08388%)	⬇️
server/remote_signer.go	0.00000% <0.00000%> (ø)

... and 2 files with indirect coverage changes

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Files with missing lines	Coverage Δ
core/livepeernode.go	60.22099% <ø> (ø)
cmd/livepeer/starter/flags.go	0.00000% <0.00000%> (ø)
cmd/livepeer/starter/starter.go	21.90476% <16.00000%> (-0.08388%)	⬇️
server/remote_signer.go	0.00000% <0.00000%> (ø)

... and 2 files with indirect coverage changes

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Some initial thoughts as I went through this PR:

• We should add an Authorization secret to protect the remote signer endpoints. WDYT?

• I think we should add a standard port at top of starter.go similar to other modes for go-livepeer

• I think the signed Gateway address to send in GetOrchestrator request should be a GET request since no payload is required. We probably should cache this on the Gateway as well since it doesn't change and would lower the requests to the signer service.

  • Seems like this request should be sent at Gateway startup and the Gateway should fail to startup if the request fails.

• Need a POST request endpoint on remote signer expecting json payload of { nonces: [list of nonces to use], ticket_params: [orch-ticket-params] that returns list of "tickets" which is just a list of TicketSenderParams that is nonce and signature for all requested nonces.

• Where should the Sender and Sender Monitor live? I think the Gateway (or local SDK) should still be responsible for this since it is directly communicating with the Orchestrator. Keeping the remote signer lightweight will assist in payment services being built since it does one thing which is use a ethereum address to sign a specific payload.

• When we settle on the implementation we should add a doc/remote_signer.md for basic instructions to run Remote Signer.