rpc: add support for protobuf-encoded messages

[tony-iqlusion]

Adds initial support for the Protobuf-based RPC protocol used by Tendermint v0.34.

[codecov-io]

Codecov Report

Merging #201 into develop will decrease coverage by 0.81%.
The diff coverage is 4.54%.

@@             Coverage Diff             @@
##           develop     #201      +/-   ##
===========================================
- Coverage    22.68%   21.86%   -0.82%     
===========================================
  Files           67       67              
  Lines         2597     2694      +97     
===========================================
  Hits           589      589              
- Misses        2008     2105      +97     

Impacted Files	Coverage Δ
src/connection/secret_connection.rs	72.67% <ø> (ø)
src/error.rs	13.04% <0.00%> (-1.25%)	⬇️
src/rpc.rs	0.00% <0.00%> (ø)
src/session.rs	0.00% <0.00%> (ø)
src/connection/secret_connection/protocol.rs	48.83% <100.00%> (ø)

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