Add codecov coverage

[codejaeger]

Description

This PR adds code coverage using Codecov as mentioned in #420 . Currently being generated using the Travis CI pipeline.

Demo

References

• https://docs.codecov.io/docs
• https://docs.codecov.io/docs
• https://github.com/boostorg/beast

Tasklist

• Add test case(s)
• Ensure all CI builds pass
• Review and approve

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@df8068f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #532   +/-   ##
==========================================
  Coverage           ?   77.81%           
==========================================
  Files              ?      110           
  Lines              ?     4367           
  Branches           ?        0           
==========================================
  Hits               ?     3398           
  Misses             ?      969           
  Partials           ?        0           

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[mloskot]

LGTM, Thanks!

[mloskot]

In near future, I'm hoping to migrate to GitHub Actions, then this script should be re-usable there too.