[api] remove the XpanId and PanId structs

[wgtdkp]

It's an overkill to define a dedicated class/struct for a simple uint16_t
just for pretty print to json in HEX string.

The library interface should prefer native/primitive data types for better
interoperability with other languages such as java and Objective-C

This commit basically reverts the XpanId and PanId changes introduced in #202

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.08696% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 72.83%. Comparing base (79d1ddc) to head (143953c).
Report is 1 commits behind head on main.

❗ Current head 143953c differs from pull request most recent head 5cb6cc8. Consider uploading reports for the commit 5cb6cc8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #264      +/-   ##
==========================================
- Coverage   72.88%   72.83%   -0.05%     
==========================================
  Files          72       72              
  Lines        7500     7476      -24     
==========================================
- Hits         5466     5445      -21     
+ Misses       2034     2031       -3     

Files	Coverage Δ
src/app/commissioner_app.hpp	100.00% <ø> (ø)
src/app/json.cpp	81.26% <100.00%> (-0.18%)	⬇️
src/app/ps/registry.cpp	89.67% <100.00%> (+0.05%)	⬆️
src/app/ps/registry_entries.cpp	98.30% <100.00%> (+1.12%)	⬆️
src/app/ps/registry_entries.hpp	100.00% <ø> (ø)
src/common/error.cpp	96.00% <100.00%> (ø)
src/common/utils.hpp	100.00% <100.00%> (ø)
src/library/network_data.cpp	74.64% <ø> (-4.74%)	⬇️
src/app/cli/interpreter.cpp	79.40% <75.00%> (-0.07%)	⬇️
src/app/ps/persistent_storage_json.cpp	58.94% <50.00%> (ø)
... and 5 more

[wgtdkp]

@abtink I am not sure if you are willing to review the ot-commissioner changes :)

In case no, please feel free to remove yourself from the pending list, I can wait for rongli :)