Support for rustls-native-certs

[charlespierce]

Inspired by the rustls-tls-native-roots feature in Reqwest, would this project be open to a feature flag to enable pulling root certificates from the native store via rustls-native-certs?

It's possible today to add them manually using the add_root_certificate method, however that adds a couple of complications:

• The consumer needs to make sure to exactly match the rustls-native-certs version to the rustls version used transitively within attohttpc
• The consumer needs to create helpers to make sure that all requests go through a RequestBuilder that adds those certs.
• Some native stores hold old certs with invalid formats, which are rejected when they're added using add_root_certificate (see a comment in the reqwest implementation)

Having the ability to use the native store built in to attohttpc would alleviate all of these and allow consumers to opt-in easily via a feature flag.

I'm happy to take on creating a PR, if it's a feature you're open to adding.

[sbstp]

Yeah sure!