Implement ExposedThing functionality

[JKRhb]

This PR will add support for exposing Things to the project, starting with the HTTP binding.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 6.61479% with 240 lines in your changes missing coverage. Please review.

Project coverage is 57.67%. Comparing base (fc3d0a6) to head (748790e).

Files	Patch %	Lines
lib/src/core/implementation/exposed_thing.dart	0.79%	125 Missing ⚠️
lib/src/binding_http/http_server.dart	2.40%	81 Missing ⚠️
lib/src/core/implementation/servient.dart	29.03%	22 Missing ⚠️
lib/src/binding_http/http_extensions.dart	0.00%	10 Missing ⚠️
lib/src/binding_coap/coap_server.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
- Coverage   63.34%   57.67%   -5.67%     
==========================================
  Files          78       79       +1     
  Lines        2275     2495     +220     
==========================================
- Hits         1441     1439       -2     
- Misses        834     1056     +222     

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