feat: allow overriding route hints in BOLT11 invoice creation

[ajaysehwal]

Summary

Adds route_hints_override to Bolt11InvoiceParameters, allowing callers of ChannelManager::create_bolt11_invoice to control which route hints are embedded in a BOLT11 invoice.
Today, create_bolt11_invoice always derives route hints automatically from local channels via sort_and_filter_channels. That works for the common case, but leaves no way to:

• supply custom or multi-hop route hints,
• pin a specific inbound channel (e.g. a preferred LSP),
• or omit route hints entirely when the payer already knows how to reach the node.
  This change adds an optional override while preserving existing default behavior.

Changes

• Add route_hints_override: Option<Vec<RouteHint>> to Bolt11InvoiceParameters.
• Update create_bolt11_invoice to branch on the new field:
  • None — existing behavior: select hints from eligible local channels via sort_and_filter_channels.
  • Some(vec![]) — include no route hints in the invoice.
  • Some(hints) — use the provided hints as-is.
• Import RouteHint in channelmanager.rs.

API

let params = Bolt11InvoiceParameters {
    amount_msats: Some(10_000),
    route_hints_override: Some(vec![custom_hint]),
    ..Default::default()
};
let invoice = channel_manager.create_bolt11_invoice(params)?;
When route_hints_override is unset (the default), behavior is unchanged from before this PR.

Backward compatibility
Fully backward compatible. route_hints_override defaults to None, and existing callers using Bolt11InvoiceParameters::default() or ..Default::default() continue to get automatic route hint selection with no code changes.

[kaloudis]

Logic looks correct. The three states (None / Some(vec![]) / Some(hints)) are handled cleanly and default behavior is preserved. Note the CI failures are all pre-existing on the base branch (lsps7 rustfmt/clippy, runner issues); nothing flags this diff, and the stable builds pass.

Could you add test coverage for the three states? e.g. None -> auto hints unchanged, Some(vec![]) -> no private routes in the invoice, Some(custom) -> invoice contains exactly the provided hints.