Now, callers have more control over the set of roots.
Note that, due to cargo unification, other dependencies in the
dependency tree might enable rustls-tls-webpki-roots
or rustls-tls.
This will affect connections initiated by code that explicitly
enabled rustls-tls-manual-roots.
So for now, the choice is done once per entire cargo
dependency graph. If people want more precise control
over things, they can add methods that allow controlling
this on a per-connection level. Even if such methods
are available, the *-manual-roots feature will still be
helpful with eliminating the webpki-roots dependency
for those cargo graphs where there is no unification.
To allow for the default-tls to change to a different backend by
default, this adds a new `native-tls` optional feature. Any TLS feature
that was only available using native-tls now requires the `native-tls`
feature to be enabled.
