Custom tlvs

[Billa05]

Closes #217.

Exposes CustomTlvRecord through the gRPC API (LDK Node already supports it under the hood):

• New types.CustomTlvRecord { uint64 type_num; bytes value; }
• repeated custom_tlvs field on SpontaneousSendRequest
• repeated custom_records field on PaymentReceived and PaymentClaimable
• --custom-tlv <type_num>:<hex_value> flag (repeatable) on ldk-server-cli spontaneous-send

Strictly wire-additive, no new dependencies. Unit tests cover the proto <-> ldk-node conversions and the PaymentClaimable proto build, e2e-tests::test_cli_spontaneous_send_with_custom_tlvs drives the full CLI round-trip.

Note: the receive path round-trips odd-type TLVs only. LDK Node's spontaneous-receive uses claim_funds(preimage) rather than claim_funds_with_known_custom_tlvs, so BOLT 4 rejects even-type TLVs at the receiver. Inherited from the pinned ldk-node revision, this surface will support even types automatically once upstream changes that.

[ldk-reviews-bot]

I've assigned @benthecarman as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.