[fix] Fix issue where cert chain is not taken into account in mTLS authentication

[massakam]

Motivation

The C++ client has the same mTLS authentication issue as apache/pulsar#23644. That is, if a client's certificate is not directly signed by a root CA, but is signed by an intermediate CA, mTLS authentication will fail.

Modifications

• When loading a client cert, use ssl::context::use_certificate_chain_file instead of ssl::context::use_certificate_file.
• Added a test to check whether a PEM file containing a client cert and intermediate CA cert can be loaded and successfully connected to a broker. When issuing the intermediate CA cert, I did not know the private key of the root CA, so I reissued the root CA cert.

Verifying this change

• Make sure that the change passes the CI checks.

Documentation

• doc-not-needed

[hrsakai]

LGTM

[BewareMyPower]

3.7.0 is currently blocked at the macOS release process, so I think it can be included in 3.7.0