feat(binding_coap): add support for PEM root certificates

[JKRhb]

This PR adds some improvements to the handling of DTLS within the CoAP binding. To be able to merge this PR, a new version of the coap package has to be released first, therefore I am keeping this one a draft for now.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.23%. Comparing base (b180edb) to head (c3c1e5f).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/binding_coap/coap_client.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
- Coverage   63.26%   63.23%   -0.03%     
==========================================
  Files          79       79              
  Lines        2311     2312       +1     
==========================================
  Hits         1462     1462              
- Misses        849      850       +1     

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[JKRhb]

As a new version of coap as been released in the meantime, this one can be merged now :)