Report `PaymentStatus` using the variant name, not the `i32` discriminant of the variant

[tankyleo]

Probably part of #14

PaymentStatus is defined in ldk-server-protos/src/types.rs:

/// Represents the current status of a payment.
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "serde", serde(rename_all = "snake_case"))]
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, ::prost::Enumeration)]
#[repr(i32)]
pub enum PaymentStatus {
 /// The payment is still pending.
 Pending = 0,
 /// The payment succeeded.
 Succeeded = 1,
 /// The payment failed.
 Failed = 2,
}

and we print this struct serialized with serde_json::to_string_pretty in ldk-server-cli/src/main.rs

fn handle_response_result<Rs, Js>(response: Result<Rs, LdkServerError>)
where
 Rs: Into<Js>,
 Js: Serialize + std::fmt::Debug,
{
 match response {
 Ok(response) => {
 let json_response: Js = response.into();
 match serde_json::to_string_pretty(&json_response) {
 Ok(json) => println!("{json}"),
 Err(e) => {
 eprintln!("Error serializing response ({json_response:?}) to JSON: {e}");
 std::process::exit(1);
 },
 }
 },
 Err(e) => {
 handle_error(e);
 },
 }
}