feat: implement directory discovery

[JKRhb]

This PR adds support for the directory discovery method to the library.

Resolves #9.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.89 ⚠️

Comparison is base (bef2416) 60.41% compared to head (d7fe0a6) 59.52%.

❗ Current head d7fe0a6 differs from pull request most recent head 8f470e8. Consider uploading reports for the commit 8f470e8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   60.41%   59.52%   -0.89%     
==========================================
  Files          70       70              
  Lines        2185     2221      +36     
==========================================
+ Hits         1320     1322       +2     
- Misses        865      899      +34     

Impacted Files	Coverage Δ
lib/src/binding_coap/coap_client.dart	1.89% <0.00%> (-0.08%)	⬇️
lib/src/binding_http/http_client.dart	48.59% <ø> (-3.88%)	⬇️
lib/src/binding_mqtt/mqtt_client.dart	0.00% <ø> (ø)
lib/src/core/thing_discovery.dart	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes

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[JKRhb]

Superseded by #84.