feat(tools): Support customizing Tool input description

[davidmigloz]

Eg.

final class CalculatorTool extends Tool<ToolOptions> {
  CalculatorTool()
      : super(
          name: 'calculator',
          description: 'Useful for getting the result of a math expression that could be executed by a simple calculator.',
          inputDescription: 'A valid mathematical expression to evaluate',
        );

  // ...
}

Or from a function:

final myTool = Tool.fromFunction(
  name: 'myTool',
  description: 'My tool description...',
  inputDescription: 'Description of the input of my tool',
  func: (
    final String toolInput, {
    final ToolOptions? options,
  }) => //...,
);