[wip] Forward compatibility for definition schema

[lmolkova]

This is stacked on #1365, delta: ad407f4

Changes:

• add unexpected field detection in definition schema (always log warn on unknown ones with file_format: definition/2)
• add support for definition/2.{x} format, make behavior consistent with resolved/manifest (fail on unknown when minor version is known, log-warn on future).

[codecov]

Codecov Report

❌ Patch coverage is 94.18605% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.8%. Comparing base (2a09a07) to head (ad407f4).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/weaver_semconv/tests/forward_compat.rs	88.4%	6 Missing ⚠️
crates/weaver_semconv/src/manifest.rs	83.3%	4 Missing ⚠️
crates/weaver_semconv/src/unexpected_fields.rs	92.0%	4 Missing ⚠️
crates/weaver_resolver/src/loader.rs	85.7%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1422     +/-   ##
=======================================
+ Coverage   82.4%   82.8%   +0.3%     
=======================================
  Files        120     128      +8     
  Lines      10111   10701    +590     
=======================================
+ Hits        8336    8865    +529     
- Misses      1775    1836     +61     

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