Merge metadata props in rewriter

[gramalingam]

Introduce basic infrastructure for merging metadata props (for use in rewriter/optimizer etc.) A basic version added to rewriter.

TODO:

• Allow user control over this: should this be configurable at the level of a RewriteRuleSet? Or, perhaps at a global level (given that ORT fusions uses a number of rewrite-rule-sets for various reasons)?
• This line should also be factored out or made user-controllable in some fashion. Otherwise, the metadata gets lost anyway.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.12%. Comparing base (93783ee) to head (9ffcdb6).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/utils/metadata_merger.py	82.05%	5 Missing and 2 partials ⚠️
onnxscript/rewriter/_rewrite_rule.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2682      +/-   ##
==========================================
- Coverage   70.46%   70.12%   -0.34%     
==========================================
  Files         224      225       +1     
  Lines       26812    27000     +188     
  Branches     2686     2711      +25     
==========================================
+ Hits        18893    18935      +42     
- Misses       6987     7131     +144     
- Partials      932      934       +2     

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[justinchuby]

Can have a test?