Replace howsmyssl.com with letsencrypt.org in the https_x509_bundle
example. howsmyssl.com is a third-party server that is frequently
unreachable from CI, causing flaky test failures. letsencrypt.org
chains to the same ISRG Root X1 CA, so the custom certificate bundle
validation coverage is identical.
(cherry picked from commit d6596eff3a)
Adds the ESP certificate bundle feature that enables users to bundle a
root certificate bundle together with their application.
Default bundle includes all Mozilla root certificates
Closes IDF-296