feat: Add support for dialogTheme in StacTheme

[divyanshub024]

Description

We need to add support for customizing dialog styles via the theme system in Stac. This involves introducing a new StacDialogThemeData class and integrating it within the existing StacTheme class.

Requirements

• Create a new class StacDialogThemeData to encapsulate dialog-related theming.
• Update StacTheme to include a dialogTheme property of type StacDialogThemeData.

Additional Context

Reference: Flutter’s DialogTheme class

[divyanshub024]

@Potatomonsta assigning this issue to you.

[Potatomonsta]

@divyanshub024 sure, will take this up