[rest] add coprocessor version string API

[puddly]

We'd like to access the current NCP/RCP coprocessor firmware version string from the REST API. I've added a new API endpoint under /node/coprocessor/version that fetches it with GetCoprocessorVersion().

[bukepo]

Perhaps we can have a refactor to these resource handlers to return whether the request is successfully processed so that this error can be responded in one place.

[puddly]

Do you mean in this specific PR or in general?

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 45.92%. Comparing base (2b41187) to head (baf9f4e).
Report is 885 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rest/resource.cpp	85.71%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2602      +/-   ##
==========================================
- Coverage   55.77%   45.92%   -9.86%     
==========================================
  Files          87      106      +19     
  Lines        6890    12608    +5718     
  Branches        0      914     +914     
==========================================
+ Hits         3843     5790    +1947     
- Misses       3047     6502    +3455     
- Partials        0      316     +316     

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