Add the indexed version of the register-level gateway

Currently register-level gateways only allow a specific address to be accessed.

It would be useful to extend the gateway so that a RLG can point to a specific address range (start address, size), instead of a single address, and provide a runtime-defined index inside that range.

In this way, the access can be partially dynamic (index), but the ACL associated with it is still static (start address, size).

Start by locating the register-level gateway implementation and the ACL handling for its current single-address access. Define how a gateway records a start address and size, how the runtime index selects an address within that range, and how to verify that the static ACL still covers every access.

Written by the indexing model from the issue text.