feat: Allow the api_key_command to read from the stdin

[DenisGorbachev]

[efugier]

Hey @DenisGorbachev thank you for the contribution! Could you provide an example use case as well so I could update the doc? (or could you please directly add it?)

[DenisGorbachev]

Hey @efugier, I needed to read the Anthropic API key from OnePassword. OnePassword provides op CLI, so I needed to run op signin + op read. Note that op signin reads the password from stdin, which is why I patched smartcat to allow the api_key_command to read from stdin.

BTW, thank you for developing smartcat and adding the api_key_command option!