[pdata] Add AsReadOnly function to pcommon.Map and pcommon.Slice

[edmocosta]

Description

This PR adds the function AsReadOnly to the pcommon.Map and pcommon.Slice, which goal is to allow API users to create read-only shallow copies of those objects.

This utility is specially useful when those types are passed around, and no modification is expected to be performed on them. For example, a few OTTL functions are currently leveraging those objects mutability to set their values (see), causing problems and unexpected behavior. Entirely copying them could be an alternative to avoid that, but it's too expensive.

Testing

Unit tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.67%. Comparing base (ac520a5) to head (c1758c4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12995   +/-   ##
=======================================
  Coverage   91.67%   91.67%           
=======================================
  Files         503      503           
  Lines       27819    27825    +6     
=======================================
+ Hits        25502    25508    +6     
  Misses       1828     1828           
  Partials      489      489           

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