refactor(api): use ini crate to parse bitcoin.conf

Replace the hand-rolled INI parser in read_bitcoin_conf_credentials
with the rust-ini crate (imported as 'ini'). The function shrinks
from ~35 lines to 6 and we stop maintaining INI parsing semantics
in this repo.

api/Cargo.toml

@@ -13,6 +13,7 @@ readme = "README.md"
 
 [dependencies]
 axum = { workspace = true }
+ini = { package = "rust-ini", version = "0.21.3" }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 stealth-bitcoincore = { path = "../bitcoincore" }