Add CPFP Support with Multi-Parent Bumping and Ancestor-Aware Fee Calculation

[aagbotemi]

Objectives

Implement CPFP functionality to boost low-fee parent transactions by creating child transactions with high fee rates. This feature should support boosting multiple transactions with a single CPFP transaction and consider all unconfirmed ancestors in fee rate calculations.

Proposed Implementation

• Parent Output: Use CanonicalUnspents to select the parent’s unconfirmed output as a must_select input.
• High Fee Rate: Set a high target_feerate in SelectorParams to make the package attractive to miners.
• Anti-Fee-Sniping: Disable it to avoid delays(do not add it).
• API Extension: Add a CpFpParams struct to encapsulate parent details and fee rate.

Rationale

• Ensures the parent output is spent, linking the transactions.
• High fee rate incentivizes miners to include both.
• Disabling anti-fee-sniping avoids unnecessary delays.

Add examples/cpfp.rs to show CPFP boosting a stuck transaction.