Add support for receiving MO JSON body

[norkans7]

So far we support receiving Form and XML only

For aggregator that forward message as JSON body only we can support them with this PR changes

[codecov]

Codecov Report

Merging #614 (0474a79) into main (adb84e4) will increase coverage by 0.33%.
Report is 3 commits behind head on main.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #614      +/-   ##
==========================================
+ Coverage   74.36%   74.70%   +0.33%     
==========================================
  Files          99       99              
  Lines       13347    13462     +115     
==========================================
+ Hits         9926    10057     +131     
+ Misses       2705     2685      -20     
- Partials      716      720       +4     

Files Changed	Coverage Δ
handlers/external/external.go	89.35% <66.66%> (-1.40%)	⬇️

... and 1 file with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[rowanseymour]

If we don't need this now, I'm inclined to not make that channel type any more complicated