Add registry.go and factory.go for proxy middleware

[Meghna45-code]

Changes Made:

1. Added registry.go to manage custom proxy middleware registration.
2. Added factory.go interface for creating proxy stacks per endpoint.
3. Updated code structure to place middleware and proxy logic in proper directories.

Reason:

1. Allows modular addition of custom middleware.
2. Supports scalable proxy creation for endpoints.