Add New Bookkeeper RPC to get Node's balance snapshot

[ShahanaFarooqui]

Currently, there is no straightforward way to generate a snapshot of a node's financial state (wallet balance and channel balances) at a given point in time. This command would be useful for auditing, tax reporting, or debugging balance discrepancies.

Proposed Solution: Create a new command, e.g., bkpr-snapshot to:

• Support time granularity for periodic snapshots.
• Filter and aggregate events chronologically between start_time and end_time.
• Optionally filter by account (e.g., a specific channel or wallet).
• Output structured balance summaries per period.

Input Parameters:

• start_time (u32, optional): UNIX timestamp (in seconds) that filters events after the provided timestamp. The default is zero.
• end_time (u32, optional): UNIX timestamp (in seconds) that filters events up to and at the provided timestamp. The default is max-int.
• time_granularity (string, optional) (one of 'minutely', 'hourly', 'daily', 'weekly', 'monthly', 'yearly'): Snapshot frequency. The default is 'daily'.
• account (string, optional): If specified, filters snapshot for the specified account only.

Example Input:

lightning-cli bkpr-snapshot -k time_granularity=daily start_time=1743532200 end_time=1743704999

Example Output:

[
  {
    "period_key": "2025-04-02",
    "total_balance_msat": 175018872090,
    "wallet_balance_msat": 117499668000,
    "channel_balances": {
      "157x1x0": 551927030,
      "151x1x0": 56967277060
    }
  },
  {
    "period_key": "2025-04-03",
    "total_balance_msat": 175024926277,
    "wallet_balance_msat": 117499668000,
    "channel_balances": {
      "157x1x0": 558081217,
      "151x1x0": 56967177060
    }
  }
]