feat: Add loading and custom loader support to GooglePayButton

[hanumanteshvar07-design]

Problem
Currently, the GooglePayButton component lacks built-in support for a loading state. This results in poor UX during the payment initiation process, as users often tap the button multiple times when there is no immediate visual feedback. Developers are forced to manually overlay spinners or wrap the button in parent views to handle loading states.

Solution
This PR introduces native support for a loading state directly in the GooglePayButton component.

Key Changes
New loading Prop: A boolean that, when true, automatically disables the button and displays a centered loader overlay.
New loader Prop: Allows developers to pass a custom React component to be used as the loading indicator.
Custom Loader Component: Added a brand-aligned, high-fidelity rotating arc spinner as the default loading indicator. This matches Google Pay's modern design language.
Modularity: Moved the loading logic into a dedicated src/Loader.tsx component and exported it via the main index, making it available for reuse throughout the application.
Theme Support: The default loader is theme-aware, automatically switching between white and black based on the button's theme (Dark/Light).
Improved UX: The native button content is hidden during the loading phase (opacity: 0) to focus user attention on the progress indicator.

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