Update: Improve handling of CA certificates

[bioball]

These are changes made on top of https://github.com/apple/pkl/pull/378. Thanks to @translatenix for doing most of the work here!

Changes on top of the original PR:

• Add ability to set CA certificates as a byte array
• Add CA certificates option to message passing API
• Revert logic to treat --ca-certificate flag as either directory or file

Closes #378