feat(binding-coap): add support for client certificates

[JKRhb]

This PR will eventually add support for client certificates to the CoAP binding. For now, it is still WIP, though.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 63.20%. Comparing base (fc3d0a6) to head (4148ad5).

Files with missing lines	Patch %	Lines
lib/src/binding_coap/coap_client.dart	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   63.34%   63.20%   -0.14%     
==========================================
  Files          78       78              
  Lines        2275     2280       +5     
==========================================
  Hits         1441     1441              
- Misses        834      839       +5     

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