Set idempotency key in renew_leaf gRPC calls

[carsonp6]

Summary

• Adds x-idempotency-key gRPC metadata header to all three renew_leaf call sites (renew_node, renew_refund, renew_zero_timelock)
• Uses the node's refund transaction txid as the idempotency key, matching the approach in the JS SDK
• Extracts a reusable set_idempotency_key helper in a new metadata.rs module for use by future RPC endpoints

Motivation

We've been seeing renew_leaf failures when paying Spark invoices:

validating refresh timelock failed: leaf {leaf_id} refund transaction sequence must be less than or equal to 300, got 2000

Root cause: the SDK occasionally calls renew_leaf twice for the same leaf within milliseconds. The first call succeeds but the second fails because the leaf was already renewed. With the idempotency key, the signing operator returns the cached response from the first call instead of processing the duplicate.

[dangeross]

LGTM. Looks like just faucet errors in the tests

[roeierez]

Thanks a lot @carsonp6 !
Looks good, just a minor comment.

[JssDWt]

Looking good. Just a nit.

[JssDWt]

I took the liberty of taking over this PR.