feat: Standardize otap nullable and optional columns

[JakeDern]

Change Summary

This is a spec compliance PR overhauling the relationship between nullable and optional columns as described in #2149 across both go and rust implementations.

Major changes:

• [spec] Removed the "nullable" column from payload specification tables since "required" is sufficient
• [spec] Changed logs "time_unix_nano" field to optional because I believe this is consistent with the otlp spec
• [spec] clarified the relationship between nullable and optional columns in a few spots
• [spec] added "flags" to the spans table which was missing
• [spec] added "zero_threshold" to the exp histogram dp table which was missing
• [spec] made span links "trace_id" and "span_id" columns required
• [rust] Updated the "required" property of some fields in payloads.rs to match the spec
• [go] Updated the optional and required metadata across many fields to match the spec

What issue does this PR close?

• Closes Proposal: Standardize the relationship between nullable and optional columns #2149

How are these changes tested?

Are there any user-facing changes?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.17%. Comparing base (b4867bf) to head (d00d4ac).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2450      +/-   ##
==========================================
- Coverage   88.18%   88.17%   -0.01%     
==========================================
  Files         604      604              
  Lines      214589   214724     +135     
==========================================
+ Hits       189235   189342     +107     
- Misses      24828    24858      +30     
+ Partials      526      524       -2     

Components	Coverage Δ
otap-dataflow	90.11% <100.00%> (-0.02%)	⬇️
query_abstraction	80.61% <ø> (ø)
query_engine	90.74% <ø> (ø)
syslog_cef_receivers	∅ <ø> (∅)
otel-arrow-go	52.45% <100.00%> (+0.01%)	⬆️
quiver	91.94% <ø> (ø)

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[lquerel]

LGTM.

[JakeDern]

I think the test failures here are also unrelated, but it still makes me a bit uncomfortable as this is a riskier change.