v0.6.0-rc.1

Add Replace-By-Fee (RBF) and Child-Pays-For-Parent (CPFP) functionality
to allow users to bump fees on stuck transactions.

• Add bump_fee_by_rbf to replace transactions with higher fee versions
• Add accelerate_by_cpfp to create child transactions that pay for parent
• Add calculate_cpfp_fee_rate helper for automatic fee calculation
• Add new error variants for transaction fee bumping operations
• Expose methods through OnchainPayment API
• Add UniFFI bindings for RBF/CPFP functionality

RBF allows replacing an existing unconfirmed transaction with a new version
that pays a higher fee. CPFP allows accelerating a transaction by spending
one of its outputs with a high-fee child transaction.