Add rustls-tls-manual-roots feature to allow callers to specify roots

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.

.github/workflows/ci.yml

@@ -55,6 +55,7 @@ jobs:
           - windows / stable-i686-gnu
           - "feat.: default-tls disabled"
           - "feat.: rustls-tls"
+          - "feat.: rustls-tls-manual-roots"
           - "feat.: native-tls"
           - "feat.: default-tls and rustls-tls"
           - "feat.: cookies"
@@ -103,6 +104,8 @@ jobs:
             features: "--no-default-features"
           - name: "feat.: rustls-tls"
             features: "--no-default-features --features rustls-tls"
+          - name: "feat.: rustls-tls-manual-roots"
+            features: "--no-default-features --features rustls-tls-manual-roots"
           - name: "feat.: native-tls"
             features: "--features native-tls"
           - name: "feat.: default-tls and rustls-tls"