1. Enhanced SBE getField Benchmark and Optimized Field Lookup

This PR makes two key improvements:

1. Completely rewrites the SBE getField benchmark to test various scenarios with a policy-based approach:

• Document shapes (single field, many fields with target first/last)
• Value types (small int32, 1KB string)
• Field name lengths (short, medium, long)
• Presence patterns (always hit, always miss, 90% hit/10% miss)

2. Optimizes the getField implementation in values/bson.h:

• First checks field name length equality & first char
• Uses memcmp for full string comparison