Support reading multiple X.509 certificates from a single PEM file

Suggested release note:

When SecurityConfig.Builder.trustCertificate(Path) is given a file containing more than one certificate, it now trusts all the certificates instead of just the first one.

Likewise, the result of SecurityConfig.decodeCertificates(List<String>) now includes all certificates in each string, not just the first certificate in each string.

Let SecurityConfig.Builder.trustCertificate(Path) read multiple certificates from a PEM file, instead of just the first one.