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

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

Define 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).