Add terms extension specification

[CameronWhiteside]

Summary

Adds the terms extension, providing a channel for servers to communicate usage rights, obligations, and settlement terms alongside the PaymentRequired response.

• Scheme- and network-agnostic — works with exact, batch-settlement, or any future scheme
• Supports multiple formats: uri, markdown, plaintext, json
• Optional version field for change detection

Split from #1145 for separate review per discussion with @CarsonRoscoe.

A note on scope

This extension defines how terms are communicated — not how they are consented to. The spec takes the implicit approach: terms are presented, and proceeding with payment implies awareness. Whether a network requires explicit consent (e.g., tying terms acceptance to a signed SIWX message) is a per-network or future-extension concern. The priority is establishing a standard channel for terms to be discoverable and machine-readable.

What's Included

File	Description
specs/extensions/terms.md	Terms extension specification

Tests

None — spec-only, no SDK implementation.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• SKIPPED added a changelog fragment for user-facing changes (docs-only changes can skip)

[vercel]

@CameronWhiteside is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.