feat: UTXO splitting for large TXs & Payment Cancel API

[martinserts]

Description

This PR introduces two main improvements:

1. Automated UTXO Splitting (Chained Transactions)

• If a transaction requires more UTXOs than allowed by MAX_INPUT_COUNT_PER_TX, the system will now split the transaction.
• It creates a self-transfer (consolidation) transaction first, followed by a transaction to the intended recipient using the unconfirmed outputs (0-conf).

2. Payment Cancellation API

• Adds a new endpoint to allow users to cancel a pending payment.

New Environment Variables:

• MAX_INPUT_COUNT_PER_TX: (Integer) The maximum number of inputs allowed in a single transaction before splitting occurs. Default: 400
• CONFIRMATION_CHECKER_REQUIRED_CONFIRMATIONS: (Integer) The number of confirmations required. Default: 10

Motivation and Context

Previously, if a user submitted a transaction requiring a massive number of UTXOs (making the TX size/weight too large for standard propagation or block limits), the transaction would simply fail.

This PR solves this by detecting heavy transactions and breaking them down:
Step 1: Create a transaction to self to consolidate inputs.
Step 2: immediately spend the output of Step 1 to pay the recipient (chaining the transactions).

Additionally, we needed a way to manually stop payments, hence the new Cancel API.

Breaking Changes

• None
• Requires data directory on base node to be deleted
• Requires hard fork
• Other - Please specify