Allow configuring the max GRPC reponse size to serve bigger proofs

[poszu]

Closes #385
Configurable receive message size on the GRPC gateway to allow increasing the maximum message size that the poet is able to serve via the REST proxy. The send message size on the GRPC service is already unlimited by default.

Set the default to 8MiB.

[codecov]

Codecov Report

Merging #395 (4d0e2ec) into develop (fb07952) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           develop    #395     +/-   ##
=========================================
- Coverage     74.8%   74.8%   -0.1%     
=========================================
  Files           23      23             
  Lines         1898    1899      +1     
=========================================
  Hits          1421    1421             
  Misses         359     359             
- Partials       118     119      +1     

Files Changed	Coverage Δ
server/config.go	77.7% <ø> (ø)
server/server.go	80.3% <100.0%> (+1.3%)	⬆️

... and 1 file with indirect coverage changes