[UR] Add PI changes to support multi-device contexts.

[veselypeta]

intel/llvm#6446 introduces several changes to the PI interface - we should reflect these changes in unified runtime. This PR is still in-review so some changes may still occur.

Key Changes:

• new entry-point piextGetMemoryConnection
• new enum pi_memory_connection
• additional parameter hDevice is passed to piMemBufferCreate & piMemImageCreate

[kbenzie]

I think this is a duplicate of #53