Route pay_any LNURL fetches through the shared HTTP client

`HTTPHrnResolver::new()` builds its own `reqwest::Client` and
ignores any proxy config, so LN-Address pays via /pay were
reaching the LNURL callback host directly even when
`socks_proxy` was set. /getbalance's max_sendable path was
fixed in the previous commit; this is the matching fix for
the actual send path.

Expose `MdkClient::http_client()` and pass that shared client
to `HTTPHrnResolver::with_client`. No new dependency; the
client was already built in `MdkClient::new()` and handed to
`MdkApiClient`.

Possible only because the reqwest 0.13 bump aligned mdkd with
bitcoin-payment-instructions; pre-bump the two crates pinned
incompatible reqwest majors and `with_client` rejected ours.

Author: amackillop

src/daemon/api/pay_any.rs

@@ -62,7 +62,7 @@ pub async fn handle_pay(state: AppState, req: &PayRequest) -> Result<PayResponse
     };
 
     let network = state.node.config().network;
-    let resolver = HTTPHrnResolver::new();
+    let resolver = HTTPHrnResolver::with_client(state.mdk_client.http_client().clone());
     let instructions = PaymentInstructions::parse(req.destination.trim(), network, &resolver, true)
         .await
         .map_err(map_parse_error)?;

src/mdk/client.rs

@@ -147,6 +147,14 @@ impl MdkClient {
         self.lsp_pubkey
     }
 
+    /// Shared `reqwest::Client` honouring `socks_proxy`. Hand to
+    /// any HTTP-using crate (e.g. `HTTPHrnResolver::with_client`)
+    /// rather than building a fresh `reqwest::Client` that would
+    /// bypass the proxy.
+    pub fn http_client(&self) -> &Client {
+        &self.http_client
+    }
+
     /// Best-effort estimate of the largest amount that can flow out
     /// over Lightning right now, with routing-fee headroom subtracted.
     /// Recomputed from `node.list_channels()` on every call so the