MessageBuffer.Factory and JSON extensions

[BalassaMarton]

Added Factory stub to MessageBuffer
Added missing checks to MessageBuffer methods

[codecov]

Codecov Report

Merging #237 (e36257a) into main (7ec6dfd) will increase coverage by 0.44%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   75.44%   75.89%   +0.44%     
==========================================
  Files         135      135              
  Lines        4391     4398       +7     
  Branches      138      138              
==========================================
+ Hits         3313     3338      +25     
+ Misses       1077     1059      -18     
  Partials        1        1              

Impacted Files	Coverage Δ
src/messaging/dotnet/src/Core/MessageBuffer.cs	78.26% <92.30%> (+22.44%)	⬆️
...ing/dotnet/src/Core/MessageBufferJsonExtensions.cs	100.00% <100.00%> (ø)
...ing/dotnet/src/Core/MessageRouterJsonExtensions.cs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes