Make it possible to provide multiple files to --ssl-extra-ca-certs

[spkane]

There are environments where almost everything is generated dynamically, and it is easy to store each required CA in an individual file. At the same time, it is challenging to combine them into a single file due to the lack of a way to run any commands on the file system. In our case, we are using Testkube in a Kubernetes cluster and have a postman collection that needs two different CA certs available, but it is VERY tricky to have access to them in a combined state. However, if we could point newman to both of the individual CA cert files, then this would not be an issue.

Version and environment information:

1. Newman Version (can be found via newman -v): 6.0.0
2. OS details (type, version, and architecture): Linux, Ubuntu, AMD64
3. Are you using Newman as a library, or via the CLI? CLI
4. Did you encounter this recently, or has this bug always been there: Always
5. Expected behaviour: I'd like to be able to either pass in the --ssl-extra-ca-certs argument multiple times or pass in a comma-separated list of file paths as a value.

Steps to reproduce the problem:

1. Create 2 or more CA pem files in the filesystem, that are required by a postman test.
2. Try to load them all and run the test in a single command.