[explorer/rest] feat: Added moasic and moasic listing endpoints

[AnthonyLaw]

Problem: Missing mosaic and mosaic listing endpoints in the rest service.
Solution:

• Added /mosaic/<name> endpoint for query root or sub namespace
• Added /mosaics endpoint query namespace listing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.94%. Comparing base (16edc37) to head (431a4f8).
⚠️ Report is 25 commits behind head on explorer/nem-backend.

Additional details and impacted files

@@                   Coverage Diff                    @@
##           explorer/nem-backend    #2069      +/-   ##
========================================================
+ Coverage                 96.91%   96.94%   +0.02%     
========================================================
  Files                       267      270       +3     
  Lines                     19360    19541     +181     
  Branches                    215      215              
========================================================
+ Hits                      18763    18944     +181     
  Misses                      591      591              
  Partials                      6        6              

Flag	Coverage Δ
explorer-rest	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
explorer/rest/rest/__init__.py	100.00% <100.00%> (ø)
explorer/rest/rest/db/NemDatabase.py	100.00% <100.00%> (ø)
explorer/rest/rest/facade/NemRestFacade.py	100.00% <100.00%> (ø)
explorer/rest/rest/model/Mosaic.py	100.00% <100.00%> (ø)

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