Gracefully handle invalid native root certificates

Before this patch, the `rustls::RootCertStore::add` method was used
to add all the root certificates found by `rustls_native_certs` crate.
This is a problem when an ancient or invalid certificate is present
in the native root store. `rustls` documentation says the following:

> This is suitable for a small set of root certificates that
> are expected to parse successfully. For large collections of
> roots (for example from a system store) it is expected that
> some of them might not be valid according to the rules `rustls`
> implements. As long as a relatively limited number of certificates
> are affected, this should not be a cause for concern. Use
> `RootCertStore::add_parsable_certificates` in order to add as many
> valid roots as possible and to understand how many certificates have
> been diagnosed as malformed.

With this patch, `RootCertStore::add_parsable_certificates` is used
instead for maximal compability with system store.

> Parse the given DER-encoded certificates and add all that can be
> parsed in a best-effort fashion.
>
> This is because large collections of root certificates often include
> ancient or syntactically invalid certificates.

src/tokio/rustls.rs

@@ -49,11 +49,13 @@ where
                     {
                         use real_tokio_rustls::rustls::Certificate;
 
-                        for cert in rustls_native_certs::load_native_certs()? {
-                            root_store
-                                .add(&Certificate(cert.0))
-                                .map_err(TlsError::Rustls)?;
-                        }
+                        let native_certs = rustls_native_certs::load_native_certs()?;
+                        let der_certs: Vec<Vec<u8>> =
+                            native_certs.into_iter().map(|cert| cert.0).collect();
+                        let total_number = der_certs.len();
+                        let (number_added, number_ignored) =
+                            root_store.add_parsable_certificates(&der_certs);
+                        log::debug!("Added {number_added}/{total_number} native root certificates (ignored {number_ignored})");
                     }
                     #[cfg(all(
                         feature = "tokio-rustls-webpki-roots",