with_extra_roots extension

[conradludgate]

Currently to use extra roots I need to use the following setup:

builder.dangerous()
    .with_custom_certificate_verifier(Arc::new(Verifier::new_with_extra_roots(extra_roots)?))

There exists the ext trait which could be extended to support:

builder.with_platform_verifier_and_extra_roots(extra_roots)?

This avoids needing to justify the use of dangerous when setting up TLS. This mirrors the non-dangerous with_root_certificates method.

[djc]

Sure, want to submit a PR?