Topic extensions for Message Router client

[BalassaMarton]

Added the Topic extension method for obtaining an IAsyncObservable for a given topic.

[codecov]

Codecov Report

Merging #226 (d48a405) into main (f193d6c) will increase coverage by 0.05%.
The diff coverage is 76.31%.

@@            Coverage Diff             @@
##             main     #226      +/-   ##
==========================================
+ Coverage   68.97%   69.02%   +0.05%     
==========================================
  Files         129      131       +2     
  Lines        4499     4539      +40     
  Branches      138      138              
==========================================
+ Hits         3103     3133      +30     
- Misses       1395     1405      +10     
  Partials        1        1              

Impacted Files	Coverage Δ
...dotnet/src/Client/MessageRouterClientExtensions.cs	37.50% <37.50%> (ø)
...ssaging/dotnet/src/Core/MessageRouterExtensions.cs	32.25% <40.00%> (-1.81%)	⬇️
...c/messaging/dotnet/src/Core/Internal/Disposable.cs	64.28% <64.28%> (ø)
...ng/dotnet/src/Client/Client/MessageRouterClient.cs	81.67% <83.90%> (+0.89%)	⬆️

... and 1 file with indirect coverage changes