feat: add SsFormat encoding library for SpanFE bypass

[rahul2393]

This commit adds the foundational SsFormat class that provides sortable string format (ssformat) encoding utilities. This encoding is used by Spanner for key ordering and routing.

Key features:

• Composite tag encoding for interleaved tables
• Signed/unsigned integer encoding (increasing/decreasing)
• String and bytes encoding with proper escaping
• Double encoding with proper sign handling
• Timestamp and UUID encoding
• Null value markers with configurable ordering
• TargetRange class for key range representation

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

[gemini-code-assist]

Summary of Changes

Hello @rahul2393, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces core encoding utilities for Spanner's key management and routing. It provides a robust SsFormat library for various data types and a TargetRange class for key range representation, laying the groundwork for advanced features like location-aware routing to enhance performance.

Highlights

• New SsFormat Encoding Library: Introduces the foundational SsFormat class, providing sortable string format (ssformat) encoding utilities essential for Spanner's key ordering and routing mechanisms.
• Comprehensive Encoding Support: The SsFormat library supports a wide range of data types, including composite tags for interleaved tables, signed/unsigned integers (increasing/decreasing order), strings and bytes with proper escaping, doubles with sign handling, timestamps, and UUIDs. It also includes configurable null value markers.
• New TargetRange Class: Adds a TargetRange class to represent key ranges, featuring start, limit, and approximate properties, along with a mergeFrom method for combining ranges.
• Location-Aware Routing: These changes are part of an experimental initiative to implement location-aware routing, aiming to improve latency within Spanner.

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[gemini-code-assist]

Code Review

This pull request introduces the SsFormat encoding library and the TargetRange class, which are foundational for sortable string format (ssformat) encoding utilities used by Spanner for key ordering and routing. The SsFormat class provides various encoding methods for different data types, including integers, strings, doubles, timestamps, and UUIDs, supporting both increasing and decreasing order. The TargetRange class represents a key range and includes logic for merging ranges. Overall, it is a good starting point. However, I've identified a potential correctness issue in the byte sequence encoding and some areas for improved maintainability.