Add initial implementation of SecuritesService

[oxisto]

Fixes #14

[codecov]

Codecov Report

Merging #24 (8148f61) into main (040d29a) will increase coverage by 5.07%.
The diff coverage is 73.84%.

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   66.66%   71.73%   +5.07%     
==========================================
  Files           2        4       +2     
  Lines          27       92      +65     
==========================================
+ Hits           18       66      +48     
- Misses          6       21      +15     
- Partials        3        5       +2     

Flag	Coverage Δ
unittests	71.73% <73.84%> (+5.07%)	⬆️

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Impacted Files	Coverage Δ
internal/assert/assert.go	50.00% <50.00%> (ø)
service/securities/service.go	80.39% <80.39%> (ø)

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