[mlir] Move AllocationOpInterface to Bufferize/IR/AllocationOpInterface.td.

Remove the interface from op defs in MemRefOps.td and make it an external model.

This is the first PR of many that will move bufferization-related ops, interfaces, passes to Dialect/Bufferize.
RFC: https://llvm.discourse.group/t/rfc-dialect-for-bufferization-related-ops/4712
It is still debated if the comprehensive bufferization has to be moved there as well, so for now I am just moving the "gradual" bufferization.

Differential Revision: https://reviews.llvm.org/D114147