Add GST sentence parser

[zhrbrk]

#54 Add GST sentence parser with unit test. GST is now a stand-alone feature, and also added into the GNSS features package.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.18519% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 77.78%. Comparing base (af2edeb) to head (b15db1c). Report is 1 commits behind head on main.

:exclamation: Current head b15db1c differs from pull request most recent head 61f33ee. Consider uploading reports for the commit 61f33ee to get more accurate results

Files	Patch %	Lines
src/sentences/gst.rs	87.50%	3 Missing :warning:
src/parser.rs	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   77.62%   77.78%   +0.16%     
==========================================
  Files          34       35       +1     
  Lines        1247     1274      +27     
==========================================
+ Hits          968      991      +23     
- Misses        279      283       +4     

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