feat: expose Display trait on Amount and Psbt types

[shivv23]

Description

Add #[uniffi::export(Display)] and implement the Display trait on the Amount and Psbt types, exposing them to the FFI bindings (Kotlin, Swift, Python).

Notes to the reviewers

This follows the same pattern used by other types in the codebase (FeeRate, Script, Address, Transaction, WalletEvent, etc.) that already expose Display to bindings. Part of issue #1020.

Documentation

• bitcoin — Amount and Psbt already implement Display upstream.

Changelog

changelog: added

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature