fix: bad format of rpc subscribe response

[omritoptix]

The subscribe response returned to register clients using ws had 2 issues:

1. It wasn't json-rpc 2.0 compatible
2. The response body didn't match what clients (such as keplr) were expecting.

[codecov]

Codecov Report

Merging #278 (abc5735) into main (6151a3c) will increase coverage by 0.05%.
The diff coverage is 60.86%.

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
+ Coverage   52.24%   52.29%   +0.05%     
==========================================
  Files          70       70              
  Lines       12551    12567      +16     
==========================================
+ Hits         6557     6572      +15     
  Misses       5069     5069              
- Partials      925      926       +1     

Impacted Files	Coverage Δ
rpc/json/handler.go	82.95% <33.33%> (-1.85%)	⬇️
rpc/json/service.go	64.91% <70.58%> (-0.09%)	⬇️
block/manager.go	67.58% <0.00%> (+1.28%)	⬆️

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