plugins: base: manage pad-specific data in a single structure

Create a struct (GstVaapiPadPrivate) to encapsulate the pad-specific data (i.e. buffer pool, allocator, info, caps, etc.).

Add an interface to retrieve the data struct for a given pad.

Finally, update the base plugin to use the data struct throughout the implementation.

This will enable us to easily extend the base plugin in the future to allow for N-to-1 pad subclasses (e.g. overlay/ composite).

This supersedes and is a follow-up to the discussions in !193 (comment 289263)