Conditionally use WebPKI trust anchors to validate certs

[litch]

This (finally) addresses #1 (and #24).

The interface of it is perhaps not ideal, but this is the actual logic that we need. Optionally we could set a more explicit config value instead of passing a non-path. I'd love feedback on that front.

The change is pretty trivial but it wound up being kind of a pain chasing through the relevant versions of (very old) rustls/webpki_roots to decide which versions will work elegantly together.

[grunch]

How can I test this one? I'm running this:

async fn main() {
    let address = "https://mynode.m.gcp.voltageapp.io";
    let cert_file = "";
    // let cert_file = "/path/to/tls.cert";
    let macaroon_file = "/path/to/admin.macaroon";

    // Connecting to LND requires only address, cert file, and macaroon file
    let mut client = tonic_lnd::connect(address, cert_file, macaroon_file)
        .await
        .expect("failed to connect");

    let info = client
        .lightning()
        // All calls require at least empty parameter
        .get_info(tonic_lnd::lnrpc::GetInfoRequest {})
        .await
        .expect("failed to get info");

    // We only print it here, note that in real-life code you may want to call `.into_inner()` on
    // the response to get the message.
    println!("{:#?}", info);
}

But I'm getting this response:

thread 'main' panicked at src/main.rs:18:10:
failed to connect: ConnectError { internal: ReadFile { file: "", error: Os { code: 2, kind: NotFound, message: "No such file or directory" } } }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[litch]

Do you have a file at "/path/to/admin.macaroon"?

[grunch]

Do you have a file at "/path/to/admin.macaroon"?

yes

[grunch]

I think that No such file or directory message is related to the tls.cert file because if I replace it with let cert_file = "/path/to/.polar/networks/2/volumes/lnd/alice/tls.cert";

I get this error

failed to connect: ConnectError { internal: Connect { address: "https://mynode.m.gcp.voltageapp.io", error: tonic::transport::Error(Transport, hyper::Error(Connect, Custom { kind: InvalidData, error: General("Mismatched number of certificates (Expected: 1, Presented: 2)") })) } }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace