Add a way to use your own certificate authority file/directory

Add a way to specify your own CA instead of using the one from the system. Much like the "+tls-ca" option to dig.

No files or tests are named. Start by tracing how dnsproxy configures and loads the system certificate authority, then find the option entry point for a dig-like CA file or directory setting. Done means users can provide their own CA source and TLS-based DNS connections use it instead of the system CA.

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