feat: Add isAvailable() method to AbstractCommand

[patel-vansh]

Description
This PR adds a new method in AbstractCommand.php, isAvailable(). This method can be used to check in runtime if the command should be executed or not. Many times, we need to define environment specific commands, especially dev commands that shouldn't be callable in production or testing. In this case, one can use isAvailable() method to determine whether the command should execute or not.

The run() method will first call isAvailable() before any other lifecycle methods (like initialize()/interact()/execute(), etc.). The sole purpose of this method is to allow/restrict execution of commands based on runtime environment/config/state and NOT passed arguments or options, and that's why it doesn't accept any parameters.

If a caller calls a command that is not available for particular runtime, CommandNotAvailableException is thrown.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide