tls: partially backport pull request #26415

This is a partial backport of commit f1a3968 ("tls: expose built-in
root certificates") from the master branch. The original commit adds a
new API, this commit just backports the non-visible changes to ease
backporting follow-up commits.

PR-URL: #26415
Backport-PR-URL: #29137
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>