feat(getSettings): added generic type for getSetting function

[NarHakobyan]

** PR Checklist
Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/fulls1z3/ngx-config/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

** PR Type
What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

** What is the current behavior?

Issue Number: N/A

** What is the new behavior?

** Does this PR introduce a breaking change?

[ ] Yes
[x] No

** Other information

[codecov]

Codecov Report

Merging #146 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #146   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          80     80           
  Branches       14     14           
=====================================
  Hits           80     80

Impacted Files	Coverage Δ
packages/@ngx-config/core/src/config.service.ts	100% <100%> (ø)	⬆️

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