routing: add mission control namespace support to SendPaymentV2

[Roasbeef]

This PR adds the ability for callers of SendPaymentV2 to specify a custom mission control namespace for their payments. This enables applications to maintain separate routing histories and success/failure statistics for different types of payments.

Motivation

Currently, all payments share the same mission control state, which can be problematic when an application needs different routing strategies for different payment types. For example, you might want high-value payments to use more conservative routing while micropayments could be more experimental. Or you might want to isolate routing data between different services using the same LND node.

With this change, applications can specify a namespace when making payments, and each namespace maintains its own view of channel reliability and routing success rates.

Changes

The implementation threads a new optional mission_control_namespace field through the payment flow:

• Added the field to the SendPaymentRequest RPC message
• Modified the routing layer to accept and use namespaced mission control instances
• Updated the SessionSource to dynamically select the appropriate MissionControl based on the namespace
• Connected everything through the RPC server layer

When no namespace is specified, the default mission control is used, maintaining full backward compatibility.

The changes have been tested with both unit tests for the routing logic and an integration test that verifies end-to-end functionality of payments with different namespaces.

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[Roasbeef]

Rounded things out by adding the mc namespace support to query+reset+import mc.

I think there's a gap here though: we don't persist the mc namespace for a payment, so on restart, if we resume, it'll fall back to the default mc.

[lightninglabs-deploy]

@roasbeef, remember to re-request review from reviewers when ready