Add TAIP-17: Composable Escrow workflow

- Introduces Escrow and Capture message types for holding and releasing funds
- Defines complete escrow lifecycle from acceptance through capture
- Supports both cryptocurrency assets and fiat currency denominations
- Enables payment guarantees (like credit card auth/capture) and asset swaps
- Reuses existing TAP messages (Authorize, Settle, Cancel, Reject) with TAIP-4 semantics
- Integrates with TAIP-7 policies for compliance and business requirements
- Any agent can act as EscrowAgent (exactly one required per escrow)
- Authorization model: agents acting for beneficiary control release/cancellation

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: pelle