[explorer/rest]: added mosaic endpoint

[AnthonyLaw]

What was the issue?

• Implement moasic endpoint.

What's the fix?

• Added search mosaic by name.
• Added _format_address_bytes helper function, converting bytes from db to address string.
• refactor in unit test to fix lint duplicated code issue.

[codecov]

Codecov Report

Merging #709 (5599779) into explorer/rest-namespaces-endpoint (38cea45) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@                          Coverage Diff                          @@
##           explorer/rest-namespaces-endpoint     #709      +/-   ##
=====================================================================
+ Coverage                              98.24%   98.26%   +0.01%     
=====================================================================
  Files                                    162      163       +1     
  Lines                                   6615     6668      +53     
  Branches                                 143      143              
=====================================================================
+ Hits                                    6499     6552      +53     
  Misses                                   116      116              

Flag	Coverage Δ
explorer-nodewatch	98.40% <ø> (ø)
explorer-rest	99.60% <100.00%> (+0.10%)	⬆️
faucet-authenticator	100.00% <ø> (ø)
faucet-backend	98.91% <ø> (ø)
faucet-frontend	100.00% <ø> (ø)
lightapi-python	98.35% <ø> (ø)
tools-shoestring	96.76% <ø> (ø)
tools-vanity	99.25% <ø> (ø)

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Files	Coverage Δ
explorer/rest/rest/__init__.py	98.78% <100.00%> (+0.09%)	⬆️
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)