routing+rpc: enable senders to set a custom first-hop TLV blob for `update_add_htlc`

[Roasbeef]

Protocol improvement proposals such as the endorsement bit require the ability for an implementation to set a new TLV value (so far none are used) within the update_add_htlc message. We should add such an ability to facilitate such improvement proposals.

Today the LightningPayment struct houses all payment details requested by the sender, we can set a custom TLV blob for the last hop, but not yet the first hop: https://github.com/lightningnetwork/lnd/blob/b1175514f9854b965d4716ac015ea15387c63081/routing/router.go#L2228-L2329

We should add another custom blob here, then ensure that we include it once we go to make the UpdateAddHTLC message to send to the first hop: https://github.com/lightningnetwork/lnd/blob/b1175514f9854b965d4716ac015ea15387c63081/routing/payment_lifecycle.go#L667-L696

Steps To Completion

• [ ] Add new field to LightningPayment

• [ ] Thread through context to the paymentLifecycle struct.

• [ ] Add a slim RPC cut out so we can write an itest in addition to unit tests. The itest can set a custom value for the first hop, then use the HTLCInterceptor to ensure that the value was properly set.

[dstadulis]

@GeorgeTsagk will work on this @saubyk will assign soon

[carlaKC]

👀 sign me up for some review when this is ready

[dstadulis]

fixed by https://github.com/lightningnetwork/lnd/pull/8660

[GeorgeTsagk]

Closed by https://github.com/lightningnetwork/lnd/pull/8660