Add sys library to Talaria

[engelhartrueben]

Add system library and custom error handling.

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Added functions to talaria/src/stdlib/sys.rs:

os_name -> String : the name of the agents OS
os_family -> String : the name of the agents family
username -> Result<String, Box<EvalAltResult>> : the username of the agent
hostname -> Result<String, Box<EvalAltResult>> : the host name of the agent
is_admin -> bool : checks if hermes has admin perms
reboot -> Result<(), <Box<EvalAltResult>> : reboot agent
shutdown -> Result<(), Box<EvalAltRestult>> : shutdown agent
uptime -> Result<f32, Box<EvalAltResult>> : get uptime of agent (unix only)
cpu_architecture -> String : get cpu arch
is_windows -> boolean : is agent os Windows
is_linux -> boolean : is agent os Linux
is_macos -> boolean : is agent os MacOs
is_bsd -> boolean : is agent os BSD

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Custom Errors

(@coal-rock implemented)

Rhai does not support enums, so we had to implement our own means of error handling through the standard library.

In talaria/src/stdlib/error.rs lives the enum Error where we can store our custom error types. We can retrieve the error type from a returned library function via get_error_name and the message inside via get_error_msg. New errors are to be added to this enum with an associated String value.

To import talaria/src/stdlib/error.rs for use in the standard library: use crate::stdlib::error::error::Error as script_error;

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Notes

All functions are untested on Windows and BSD.

[coal-rock]

I've gone ahead and cleaned up some of the documentation.
(I recommend you check this out if you're curious about convention/features of mdbook)

Aside from that, everything looks great!
Good work!!

LGTM 🔥