[runtime] Move sgaalloc and sgafree to transport

[iyzhang]

Context

Currently, our SharedDemiRuntime, implements the default memory runtime, so any libOSes that just allocate from the heap can allocate memory from the SharedDemiRuntime. However, for libOSes that need special memory (DPDK, raw sockets), this does not work. Currently they are still allocating memory from the transport through the libOS.

Proposed Solution

There are two ways to fix this. First is that in addition to a runtime, each libOS has a libOS-specific transport that performs low-level tasks like memory allocation. Alternatively, we can add the transport as a generic to the SharedDemiRuntime, but that would make it impossible for two libOSes with different memory allocators to share the same runtime. This may or may not be necessary/useful in the future.