Add Input Validations for Command Flags for the command `alpha generate`

[camilamacedo86]

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What do you want to happen?

Currently, the alpha generate command lacks proper input validation for its flags. This issue aims to add robust validation to ensure that invalid flag values are caught early, improving the user experience and preventing potential runtime errors.

Acceptance Criteria:

1. Implement input validations for all flags in the alpha generate command.
2. Return meaningful error messages when invalid flags are provided.

For reference see its code in: https://github.com/kubernetes-sigs/kubebuilder/pull/4180

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