feat: Add support for custom commands

[Rapsssito]

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This PR adds supports for custom RPC commands (that support any amount of data fields) so Improv can be extended by the user for custom use cases (such as cases where more credentials are required).

I added a new CUSTOM command type to the Command enum, allowing for the introduction of custom command handling. Following the same logic, I updated the ImprovCommand struct to replace ssid and password fields with a vector of data fields (but added methods to retrieve ssid and password from the data fields, ensuring backward compatibility).

[Rapsssito]

@jesserockz, I can update the website if you think adding "extended" command support is worth doing.