[explorer/rest]: added namespace and namespaces endpoint

[AnthonyLaw]

What was the issue?

• Implement namespace and namespaces endpoint.

What's the fix?

• Added search namespace by name.
• Added search namespace listing.
• Added unit test

[codecov]

Codecov Report

Merging #702 (38cea45) into dev (0f4f93e) will increase coverage by 0.02%.
Report is 3 commits behind head on dev.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #702      +/-   ##
==========================================
+ Coverage   98.22%   98.24%   +0.02%     
==========================================
  Files         161      162       +1     
  Lines        6537     6615      +78     
  Branches      143      143              
==========================================
+ Hits         6421     6499      +78     
  Misses        116      116              

Flag	Coverage Δ
explorer-nodewatch	98.40% <ø> (ø)
explorer-rest	99.50% <100.00%> (+0.31%)	⬆️
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	98.68% <100.00%> (+0.46%)	⬆️
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/Namespace.py	100.00% <100.00%> (ø)