chore!: do not expose the TD ID via AugmentedForms

[JKRhb]

As discussed in eclipse-thingweb/node-wot#1342, this PR removes the tdIdentifierthat has previously been exposed by the AugmentedForm class.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.50000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 63.20%. Comparing base (fdc099a) to head (737bc39).

Files with missing lines	Patch %	Lines
lib/src/binding_coap/coap_client.dart	0.00%	4 Missing ⚠️
lib/src/binding_http/http_client.dart	0.00%	3 Missing ⚠️
lib/src/binding_http/http_subscription.dart	0.00%	1 Missing ⚠️
lib/src/binding_mqtt/mqtt_client.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #209      +/-   ##
==========================================
- Coverage   63.23%   63.20%   -0.04%     
==========================================
  Files          79       79              
  Lines        2312     2310       -2     
==========================================
- Hits         1462     1460       -2     
  Misses        850      850              

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[JKRhb]

This PR now also simplifies the handling of the href field in AugmentedForms, as URI variables are resolved before the form is handed over to the protocol binding implementations and exposed as the regular href, making this transparent on the implementation side.