feat: add KeyRecipe and KeyRecipeCache

This commit adds the key recipe system that enables encoding of Spanner
keys from request parameters:

- KeyRecipe: Translates read/query requests into ssformat keys using
  recipes provided by the server. Supports all Spanner types including:
  - Primitive types (INT64, FLOAT64, STRING, BYTES, BOOL)
  - Date, Timestamp, and Numeric types
  - Complex types with nested field extraction
  - Ascending/descending key ordering
  - NULL handling with configurable ordering

- KeyRecipeCache: Caches KeyRecipes by table/index/query and schema
  generation. Provides efficient lookup and key computation for
  recurring request patterns.

- Golden tests: Comprehensive test suite using golden test data to
  verify key encoding correctness.

Depends on: SsFormat encoding library (PR #1)

This is part of the experimental location-aware routing for improved
latency.

Author: rahul2393