Configurable constants

[SamClercky]

This PR adds the ability to configure the user configurable constants through environment variables. This makes it easier to change them for an end-user.

Additionally, documentation in the README has been added with a short description about the meaning of all the environment variables.

Lastly, to help with better keeping track of what is happening, more fine-grained errors are added about the CCA backoff and ACK retries.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.85%. Comparing base (a583f28) to head (a6ddeca).

❗ Current head a6ddeca differs from pull request most recent head 4bfcf22

Please upload reports for the commit 4bfcf22 to get more accurate results.

Files	Patch %	Lines
dot15d4/src/csma/mod.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   75.90%   73.85%   -2.06%     
==========================================
  Files          36       36              
  Lines        5334     5447     +113     
==========================================
- Hits         4049     4023      -26     
- Misses       1285     1424     +139     

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