NYL-22: Add Contact metadata and webhook parity

[nylas-it-ops]

Summary

• add Contact metadata to response, create, and update models and builders
• add indexed metadata_pair filtering with query serialization coverage
• add contact.updated and contact.deleted webhook trigger constants and exact wire-name tests
• document metadata semantics and iCloud/Yahoo webhook support boundaries

Test plan

• ./gradlew test lintKotlin with the repository's Java 8 compiler toolchain (build successful)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.72%. Comparing base (5e0890b) to head (18a4682).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #340   +/-   ##
=========================================
  Coverage     83.72%   83.72%           
  Complexity      486      486           
=========================================
  Files            44       44           
  Lines          1358     1358           
  Branches        115      115           
=========================================
  Hits           1137     1137           
  Misses          149      149           
  Partials         72       72           

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[AaronDDM]

Good morning! Strongest of the four SDK PRs — this is the only one with a real query-serialization test, and covering both create and update builders plus the exact wire names is the right level of detail.

Three notes below, none blocking. Approving.