feat: support embedded raw body fields

[danielgtaylor]

This updates the handling of the raw body field index to match that of the body index (using a slice of field integers describing the path to the possibly embedded field). This makes it possible to use embedded structs with a RawBody field.

Fixes #652

Copilot reviewed 2 out of 2 changed files in this pull request and generated no suggestions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.76%. Comparing base (4b4221f) to head (7c050f1).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #666   +/-   ##
=======================================
  Coverage   92.76%   92.76%           
=======================================
  Files          22       22           
  Lines        4905     4910    +5     
=======================================
+ Hits         4550     4555    +5     
  Misses        308      308           
  Partials       47       47           

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