FDC3 AppDirectory implementation

[BalassaMarton]

Initial AppDirectory implementation
Work in progress with the following outstanding items:

• Load the apps from a local JSON file
• Load the apps from a JSON resource over HTTP(S)
• Load the apps from a REST API (https://fdc3.finos.org/schemas/2.1/app-directory.html)

[codecov]

Codecov Report

Merging #356 (38110a4) into main (846f612) will decrease coverage by 0.05%.
The diff coverage is 82.29%.

@@            Coverage Diff             @@
##             main     #356      +/-   ##
==========================================
- Coverage   77.29%   77.24%   -0.05%     
==========================================
  Files         156      160       +4     
  Lines        5302     5396      +94     
  Branches      233      233              
==========================================
+ Hits         4098     4168      +70     
- Misses       1203     1227      +24     
  Partials        1        1              

Files Changed	Coverage Δ
...jection/ServiceCollectionAppDirectoryExtensions.cs	0.00% <0.00%> (ø)
...tnet/AppDirectory/src/AppDirectory/AppDirectory.cs	85.18% <85.18%> (ø)
...pDirectory/src/AppDirectory/AppDirectoryOptions.cs	100.00% <100.00%> (ø)
...Directory/src/AppDirectory/AppNotFoundException.cs	100.00% <100.00%> (ø)
.../MorganStanley.ComposeUI.Testing/TaskExtensions.cs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes