Added Functionality to Identify Radio and Push Button Fields

[fuadkhan1996]

This pull request introduces two new methods to the fillable-pdf gem, enhancing its ability to identify specific form field types:

1. radio_field?(key): Determines if a given field (identified by its key) is a radio button.
2. push_button_field?(key): Determines if a given field (identified by its key) is a push button.

Use Case:

These methods provide developers with a way to programmatically check if a specific field in a PDF form is a radio button or a push button, allowing for more precise control and manipulation of form fields.

Changes:

Added radio_field?(key) method to fillable-pdf.rb. Added push_button_field?(key) method to fillable-pdf.rb.

Testing:

Ensure that these methods correctly identify radio buttons and push buttons in various PDF forms.