lsps-plugin: add cln-lsps-policy plugin for LSPS2 fee policy management

[Andezion]

Summary

cln-lsps-service calls out to two internal RPC methods to make LSPS2 policy
decisions - lsps2-policy-getpolicy and lsps2-policy-getchannelcapacity -
but no plugin in the repo registers them, so the service can't run standalone

This adds a new cln-lsps-policy binary that implements both:

• lsps2-policy-getpolicy - returns the LSP's fee parameter menu
  (Lsps2PolicyGetInfoResponse), built from configurable CLN options.
• lsps2-policy-getchannelcapacity - returns the channel capacity to open
  for a given JIT-channel buy request (Lsps2PolicyGetChannelCapacityResponse)

All policy knods are exposed as plugin options with sensible defaults, so an
LSP operator can tune fees/limits without touching code:

Option	Default
lsps2-policy-min-fee-msat	1000
lsps2-policy-proportional-ppm	0
lsps2-policy-valid-until-hours	1
lsps2-policy-min-lifetime	144
lsps2-policy-max-client-to-self-delay	2016
lsps2-policy-min-payment-size-msat	1000
lsps2-policy-max-payment-size-msat	10_000_000_000
lsps2-policy-channel-capacity-msat	10_000_000_000

Values are validated at startup (non-negative, max > min, ppm in
[0, 1_000_000]) so misconfiguration fails fast instead of silently
producing bad fee params

Test plan

• cargo build - all three binaries (cln-lsps-client,
  cln-lsps-service, cln-lsps-policy) compile cleanly
• cargo test - 84 passed, 0 failed (no regressions in existing
  lsps-plugin test suite)
• Manual end-to-end test: run cln-lsps-service + cln-lsps-policy
  together against a client and confirm lsps2.get_info /
  lsps2.buy succeed

Important

26.06 FREEZE April 30th: Non-bugfix PRs not ready by this date will wait for 26.09.

RC1 is scheduled on May 14th

The final release is scheduled for June 1st.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade

Changelog-None