- An admin-set cacerts no longer gets the CAStore cacertfile added;
Hackney drops cacerts when both are set.
- Set proxy_ssl_options too: with an HTTPS proxy, Hackney 4 uses them
for the proxy connection and falls back to certifi.
- Hackney 4 ignores the top-level insecure option and uses certifi.
Turn it into an explicit verify_none.
Hackney 4 merges given ssl_options into its defaults and keeps its own
partial_chain, which trusts any certificate in the peer chain that is in
certifi. A peer could send the emSign ECC Root CA - G3 root in its chain
and pass. Set partial_chain so certifi is never a trust anchor.
Hackney trusts its own certifi bundle, which still has the roots of the
compromised eMudhra intermediate CAs. Pass explicit ssl_options with the
CAStore bundle. Hackney drops all its TLS defaults when ssl_options are
given, so keep peer and hostname checks in the options.
