Add Auxiliary Command Server to Proto

[aphralG]

Proposed changes

Add AuxiliaryCommandServer to AgentConfig proto

message AgentConfig {
    // Command server settings
    CommandServer command = 1;
    // Metrics server settings
    MetricsServer metrics = 2;
    // File server settings
    FileServer file = 3;
    // A series of key/value pairs to add more data to the NGINX Agent instance
    repeated google.protobuf.Struct labels = 4;
    // A list of features that the NGINX Agent has
    repeated string features = 5;
    // Message buffer size, maximum not acknowledged messages from the subscribe perspective
    string message_buffer_size = 6;
    // Auxiliary Command server settings
    AuxiliaryCommandServer auxiliary_command = 7;
}

Checklist

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• 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