feat!: simplify credentials callback usage

[JKRhb]

This PR simplifies the handling of credentials provided to clients by replacing the central ClientSecurityProvider class with individual callbacks per ProtocolClientFactory. This should make the handling of credentials more flexible and improve the extensibility with new protocols of the library in the future.

[codecov-commenter]

Codecov Report

Attention: 25 lines in your changes are missing coverage. Please review.

Comparison is base (f3e39bf) 52.28% compared to head (5caeedf) 52.33%.

Files	Patch %	Lines
lib/src/binding_coap/coap_client.dart	14.28%	6 Missing ⚠️
lib/src/binding_mqtt/mqtt_client_factory.dart	0.00%	5 Missing ⚠️
lib/src/binding_http/http_client_factory.dart	0.00%	4 Missing ⚠️
lib/src/binding_http/http_client.dart	0.00%	3 Missing ⚠️
lib/src/binding_mqtt/mqtt_client.dart	0.00%	3 Missing ⚠️
lib/src/binding_coap/coap_client_factory.dart	71.42%	2 Missing ⚠️
lib/src/core/servient.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   52.28%   52.33%   +0.04%     
==========================================
  Files          70       69       -1     
  Lines        2184     2188       +4     
==========================================
+ Hits         1142     1145       +3     
- Misses       1042     1043       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.