cli: operator raft join should accept file paths for TLS parameters

[calvn]

Is your feature request related to a problem? Please describe.
The flags related to TLS parameters currently only accepts data values as literal single-line strings which can be cumbersome to provide.

Describe the solution you'd like
The vault operator raft join command should be able to accept file paths for TLS parameters.

Describe alternatives you've considered
Pass in the data as a single-line string value for each of the TLS flags.

Additional context
Related to #8753
Related to #8894