Add public method to list pending monitor updates from ChainMonitor

[dunxen]

Users have requested the feature to list pending monitor updates from ChainMonitor so this adds that.

Closes #1786

[valentinewallace]

Feel free to squash!

[codecov-commenter]

Codecov Report

Base: 90.80% // Head: 90.75% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (28695e0) compared to base (f1428fd).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 28695e0 differs from pull request most recent head 7f4ac0e. Consider uploading reports for the commit 7f4ac0e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1834      +/-   ##
==========================================
- Coverage   90.80%   90.75%   -0.06%     
==========================================
  Files          89       87       -2     
  Lines       47963    47389     -574     
  Branches    47963    47389     -574     
==========================================
- Hits        43554    43008     -546     
+ Misses       4409     4381      -28     

Impacted Files	Coverage Δ
lightning/src/chain/chainmonitor.rs	97.83% <100.00%> (+0.04%)	⬆️
lightning-invoice/src/utils.rs	94.43% <0.00%> (-0.77%)	⬇️
lightning-rapid-gossip-sync/src/processing.rs	91.48% <0.00%> (-0.53%)	⬇️
lightning-net-tokio/src/lib.rs	76.73% <0.00%> (-0.31%)	⬇️
lightning/src/routing/scoring.rs	96.61% <0.00%> (-0.29%)	⬇️
lightning/src/onion_message/blinded_route.rs	96.66% <0.00%> (-0.25%)	⬇️
lightning/src/chain/onchaintx.rs	95.01% <0.00%> (-0.22%)	⬇️
lightning/src/ln/channelmanager.rs	85.18% <0.00%> (-0.22%)	⬇️
lightning/src/util/ser.rs	91.66% <0.00%> (-0.14%)	⬇️
lightning/src/ln/payment_tests.rs	98.81% <0.00%> (-0.08%)	⬇️
... and 17 more

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[TheBlueMatt]

error[E0277]: the type `[(chain::transaction::OutPoint, std::vec::Vec<MonitorUpdateId>)]` cannot be indexed by `&chain::transaction::OutPoint`
   --> lightning/src/chain/chainmonitor.rs:820:83
    |
820 |         assert!(nodes[1].chain_monitor.chain_monitor.list_pending_monitor_updates().get(funding_txo)
    |                                                                                     --- ^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
    |                                                                                     |
    |                                                                                     required by a bound introduced by this call
    |
    = help: the trait `SliceIndex<[(chain::transaction::OutPoint, std::vec::Vec<MonitorUpdateId>)]>` is not implemented for `&chain::transaction::OutPoint`
note: required by a bound in `core::slice::<impl [T]>::get`

error[E0277]: the type `[(chain::transaction::OutPoint, std::vec::Vec<MonitorUpdateId>)]` cannot be indexed by `&chain::transaction::OutPoint`
   --> lightning/src/chain/chainmonitor.rs:824:84
    |
824 |         assert!(!nodes[1].chain_monitor.chain_monitor.list_pending_monitor_updates().get(funding_txo)
    |                                                                                      --- ^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
    |                                                                                      |
    |                                                                                      required by a bound introduced by this call
    |
    = help: the trait `SliceIndex<[(chain::transaction::OutPoint, std::vec::Vec<MonitorUpdateId>)]>` is not implemented for `&chain::transaction::OutPoint`
note: required by a bound in `core::slice::<impl [T]>::get