Add support for LibPQFactory to load root cert from the classpath

[hmendesB2C2]

I'm submitting a feature request

• bug report
• feature request

Describe the issue
At the moment the only way to provide the root certificates for LibPQFactory is by giving it the filesystem location of the file containing the root certs.
This approach has vulnerabilities because an attacker can modify the root certs file in the filesystem. Plus if the app receives the root cert from a channel other than the filesystem it forces the app to write the data to the filesystem in order for it to be used by LibPQFactory.

I propose that LibPQFactory can load the root certs from the classpath as well, basically in a manner similar to how its done by SingleCertValidatingFactory.

I'm available to raise a PR for it with a proposed implementation.