Make the SCA a user-defined actor

[adlrocha]

Potential dependencies: consensus-shipyard/ipc-subnet-actor#3, consensus-shipyard/ipc-subnet-actor#1

We want to depart from the SCA being a builtin-actor (especially as we plan the migration with Filecoin and we plan the deployment into other networks). As we move towards that goal we should:

• Extract the SCA to its own repo.
• Rename it to IPC Gateway (its new name)
• Remove builtin-actor-specific dependencies.
• Test its interoperability with user-defined subnet actors.

[adlrocha]

Being tackled in https://github.com/consensus-shipyard/ipc-subnet-actor, https://github.com/consensus-shipyard/ipc-gateway and https://github.com/consensus-shipyard/ipc-atomic-execution