Add SDK memo handling utilities

[El-swaggerito]

Description

Difficulty: Hard

Type: Feature

Summary

Add memo handling utilities for payment and transaction flows.

Current Behaviour

Memo validation and formatting may be duplicated or inconsistent across payment helpers.

Expected Behaviour

The SDK should provide reusable memo helpers for supported memo types and safe display formatting.

Suggested Implementation

Create memo types, validation helpers, display helpers, tests, and documentation.

Files or Areas Likely Affected

• src/payments/
• src/transactions/
• src/utils/
• src/types/
• tests/
• docs/

Acceptance Criteria

• Memo helper utilities are implemented.
• Supported memo types are documented.
• Invalid memo values are rejected.
• Payment helpers can use the shared memo utilities.
• Tests cover valid and invalid memo values.
• Docs explain memo limitations.

Additional Notes

This should include implementation and tests.