fix(coap-server): add missing cov:observe subprotocol

[JKRhb]

Trying to interact with a test thing using CoAP, I noticed that the forms provided in the Thing Descriptions do not include the subprotocol value cov:observe for observable properties and events. This PR tries to fix the issue by adding logic to determine if a subprotocol should be added to a form.

The implementation got a bit more complicated than initially expected because I noticed that you probably should not simply add the subprotocol to all property forms, but only those containing the op values observeproperty and/or unobserveproperty. Furthermore, I got the feeling that you should not use cov:observe with readproperty or writeproperty (which I think is currently not clearly stated in the CoAP Binding Template).

Therefore, during the TD generation process, I split the op values for read/write operations and observe/unobserve operations, and generated separate forms for the two kinds of operations. There might be some more discussion and clarification in the protocol binding document needed to decide whether cov:observe should or should not be used with forms containing readproperty and/or writeproperty. In general, the approach in this PR should work regardless of the eventual decision, though.

Besides the actual implementation, I performed some minor refactoring of the process of assigning the forms to affordances and added a test to cover all changes made.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8adf840) 77.57% compared to head (5a58d39) 77.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
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+ Coverage   77.57%   77.74%   +0.16%     
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  Files          83       84       +1     
  Lines       17332    17443     +111     
  Branches     1751     1761      +10     
==========================================
+ Hits        13446    13561     +115     
+ Misses       3850     3846       -4     
  Partials       36       36              

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[relu91]

I have some questions below before approving the PR.

[relu91]

Good to go now.

[JKRhb]

Good to go now.

Thank you! I incorporated the latest change into 55e197c, squashing the commits. Hope that was fine.