feat: pay contacts from spark #1244

Twin: synonymdev/bitkit-android#1807

What happened

A wallet whose spending balance sits in Spark can't pay a Paykit contact from it. A contact who only accepts on-chain bitcoin can't be paid from Spending at all.

Expected

Every bitcoin contact payment goes through bitkit-core's Paykit router. A Spark balance pays contacts directly over Lightning, or through a Spark withdrawal to a contact who only accepts on-chain bitcoin, with the withdrawal fee shown on the confirm screen.

See the PR that fixes it

#1191

Reproducibility

Always, on a wallet whose spending balance is in Spark.