Allow the user to supply root cert store

[stevefan1999-personal]

I happen to stumble upon here:

reqwest/src/async_impl/client.rs

Lines 508 to 511 in 3ad6e02

	#[cfg(feature = "rustls-tls-webpki-roots")]
	if config.tls_built_in_certs_webpki {
	root_cert_store.extend(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());
	}

I'm using a custom Rustls provider with RustCrypto which is in pure Rust, and I'm not using ring and do not want ring in the dependency chain.

However, I cannot control this at the moment, so I have to improvise like this:

pub fn make_client() -> Result<Client, Error> {
    let mut builder = Client::builder();
    for cert in webpki_root_certs::TLS_SERVER_ROOT_CERTS {
        builder = builder.add_root_certificate(Certificate::from_der(cert).unwrap())
    }
    builder.build()
}

Which is pretty annoying.

[stevefan1999-personal]

Partially solved by #2447 but it is limited to webpki roots. If you want other roots there is no luck atm