Support certificates for registeration

[poszu]

Closes #427

There is now an alternative way to PoW of verifying if a node is allowed to register in the open round by certificate verification.

It's considered unstable now and the poet supports both ways until it is stabilized.

If a certifier is configured (an URL + a pubkey), the poet will first try to verify the certificate. If it's not provided (for example a node that didn't update tries to register) - fallback to verifying PoW. Hence a poet that wants to support certificates should configure PoW as well until the certificate way is fully stabilized.

It returns unauthorized (HTTP 401) if the certifier is configured but the certificate is not valid, allowing the node the re-certify and retry registration with a new certificate.

[codecov]

Codecov Report

Merging #429 (1284049) into develop (fb46c6f) will increase coverage by 0.1%.
The diff coverage is 95.0%.

@@            Coverage Diff            @@
##           develop    #429     +/-   ##
=========================================
+ Coverage     77.1%   77.3%   +0.1%     
=========================================
  Files           25      25             
  Lines         1959    2010     +51     
=========================================
+ Hits          1512    1554     +42     
- Misses         330     336      +6     
- Partials       117     120      +3     

Files	Coverage Δ
registration/registration.go	78.0% <100.0%> (-0.7%)	⬇️
rpc/rpcserver.go	90.8% <100.0%> (+2.0%)	⬆️
registration/config.go	84.2% <76.9%> (-15.8%)	⬇️