Add Support for Custom Deep Link Schemes and User-Defined Handlers

[niyatinehal]

OVERVIEW

Description: - The current universal-app-opener library only supports a predefined set of platforms with hardcoded deep link handlers.
This limits its extensibility for users who need to support:

• Proprietary or internal apps with custom URL schemes
• New platforms not yet implemented in the library
• Custom deep link formats for existing platforms
• Enterprise or niche applications

Current Behavior:

• Users can only use generateDeepLink() and openLink() with the built-in platform handlers
• No API to register custom handlers or override existing ones
• Library is closed to extension without forking and modifying the source

Expected Behavior: Users should be able to:

• Register custom deep link handlers for unsupported platforms
• Override built-in handlers with custom implementations
• Provide fallback handlers for unknown URLs
• Dynamically add/remove handlers at runtime

Use Cases:

• Enterprise Apps: Support internal company apps with custom schemes
• New Platforms: Add support for emerging social media (e.g., Threads, Bluesky) before official library support
• Custom Integrations: Override default handlers for specific business logic
• Testing: Mock handlers for unit testing deep link generation
• Niche Apps: Support gaming apps, productivity tools, etc.

@mdsaban If this is something which is required then I'd like to work on it