Add optional transactionValue to TAIP-3 Transfer message

[thomasjsk]

Summary

Adds an optional transactionValue field to the Transfer message, allowing the initiating agent to
communicate the fiat equivalent value of a virtual asset transfer.

Motivation

When a transfer involves a virtual asset that is not widely traded or not yet listed on major
exchanges, receiving agents may not be able to resolve its fiat value independently. This creates
problems for:

• Travel Rule threshold determination — agents need the fiat value to decide whether Travel Rule
  obligations apply
• Amount-based policy evaluation — compliance policies often reference fiat thresholds
• Record-keeping — regulatory reporting typically requires fiat equivalents

The transactionValue field allows the originating agent to provide this value upfront, so receiving
agents don't have to rely solely on their own asset pricing services.

Changes

• TAIP-3 specification (TAIPs/taip-3.md) — Added transactionValue to the message body
  definition with amount (decimal string) and currency (ISO 4217 code). Added a test case example
  showing a transfer of a custom token with a EUR equivalent.
• JSON Schema (schemas/messages/transfer.json) — Added transactionValue object property
  referencing existing amount and iso4217CurrencyCode base types.
• TypeScript types (packages/typescript/src/tap.ts) — Added optional transactionValue field
  to the Transfer interface.

Example

{
  "asset": "eip155:1/erc20:0x1234...",
  "amount": "500",
  "transactionValue": {
    "amount": "1250.00",
    "currency": "EUR"
  }
}

Backward Compatibility

The field is fully optional. Existing implementations that do not include or handle transactionValue
are unaffected.