Switch to rustls and webpki-roots (#30025)

This change replaces OpenSSL with rustls and also the manually curated
CA certs file with webpki-roots (effectively the same thing, but as a
crate).

Generally speaking the design of the network stack is the same. Changes:

- Code around certificate overrides needed to be refactored to work with
  rustls so the various thread-safe list of certificates is refactored
  into `CertificateErrorOverrideManager`
- hyper-rustls takes care of setting ALPN protocols for HTTP requests,
  so for WebSockets this is moved to the WebSocket code.
- The safe set of cypher suites is chosen, which seem to correspond to
  the "Modern" configuration from [1]. This can be adjusted later.
- Instead of passing a string of PEM CA certificates around, an enum is
  used that includes parsed Certificates (or the default which reads
  them from webpki-roots).
- Code for starting up an SSL server for testing is cleaned up a little,
  due to the fact that the certificates need to be overriden explicitly
  now. This is due to the fact that the `webpki` crate is more stringent
  with self-signed certificates than SSL (CA certificates cannot used as
  end-entity certificates). [2]

1. https://wiki.mozilla.org/Security/Server_Side_TLS
2. https://github.com/briansmith/webpki/issues/114

Fixes #7888.
Fixes #13749.
Fixes #26835.
Fixes #29291.

python/servo/platform/linux.py

@@ -19,8 +19,7 @@ from .base import Base
 # Please keep these in sync with the packages in README.md
 APT_PKGS = ['git', 'curl', 'autoconf', 'libx11-dev', 'libfreetype6-dev',
             'libgl1-mesa-dri', 'libglib2.0-dev', 'xorg-dev', 'gperf', 'g++',
-            'build-essential', 'cmake', 'libssl-dev',
-            'liblzma-dev', 'libxmu6', 'libxmu-dev',
+            'build-essential', 'cmake', 'liblzma-dev', 'libxmu6', 'libxmu-dev',
             "libxcb-render0-dev", "libxcb-shape0-dev", "libxcb-xfixes0-dev",
             'libgles2-mesa-dev', 'libegl1-mesa-dev', 'libdbus-1-dev',
             'libharfbuzz-dev', 'ccache', 'clang', 'libunwind-dev',
@@ -31,8 +30,7 @@ DNF_PKGS = ['libtool', 'gcc-c++', 'libXi-devel', 'freetype-devel',
             'libunwind-devel', 'mesa-libGL-devel', 'mesa-libEGL-devel',
             'glib2-devel', 'libX11-devel', 'libXrandr-devel', 'gperf',
             'fontconfig-devel', 'cabextract', 'ttmkfdir', 'expat-devel',
-            'rpm-build', 'openssl-devel', 'cmake',
-            'libXcursor-devel', 'libXmu-devel',
+            'rpm-build', 'cmake', 'libXcursor-devel', 'libXmu-devel',
             'dbus-devel', 'ncurses-devel', 'harfbuzz-devel', 'ccache',
             'clang', 'clang-libs', 'llvm', 'autoconf213', 'python3-devel',
             'gstreamer1-devel', 'gstreamer1-plugins-base-devel',