systemc: Make tlm/gem5 packet conversion flexible

systemc: Make tlm/gem5 packet conversion flexible

We used to have a hard-coded packet2payload and payload2packet in the
tlm_bridge implementation. However, as the conversion is operated on
generic tlm payload, we're not able to handle information stored in any
user defined SystemC extensions.

In this CL, we add a pair of function to register extra conversion steps
between tlm payload and gem5 packet. This decouples the exact conversion
logic and enables SystemC users to register any necessary steps for
their extensions.

Change-Id: I70b3405395fed0f757f0fb7e19136f47d84ac115
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37075
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>