Add token path to Agent config Command block

[sean-breen]

Proposed changes

Allow the data plane key to be loaded from a file by adding a new configuration option token-path under the Command section of the agent config file. Also refactored some code to create new functions addTransportCredentials and validateTokenFile which are used when creating the gRPC credentials.

• If both token and token-path are set:
  • token-path will take priority
  • token field will be used as a fallback when the token-path file does not exist or is inaccessible

command:
  server:
    host: "127.0.0.1"             # Command server host
    port: 8888                    # Command server port
  tls:
    server_name: test-local-server
    ca: /tmp/ca.pem
    cert: /tmp/cert.pem
    key: /tmp/key.pem
    generate_self_signed_cert: true
  auth:
    token: "1a2b3c4d"    
    token-path: "path/to/secret-token"    # Path to a file containing the authentication token for the command sever        

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[dhurley]

Just wondering if the struct array is needed since there is only one test case

[sean-breen]

Not really needed, but I was going to leave it there in case of expansion in the future.