[bitreq] Allow adding custom root certificates

[luisschwab]

Would be nice to be able to add custom root certificates to the client.

For example, to fetch invoices from LND, I need to add its root certificate to the client. For example, reqwest does it like this:

let cert: Vec<u8> = fs::read(&self.tls_cert_path)?;
let cert: Certificate = Certificate::from_pem(&cert)?;
let client: Client = Client::builder().add_root_certificate(cert).build()?;

[tnull]

This is a duplicate of #473 and should be addressed by #502. Therefore going ahead and closing this.