feat: add ic-ref support to dfx replica command

[p-shahi]

Adds --emulator flag to the dfx replica command
Fixes an inconsistency where dfx replica was writing the replica port to .dfx/config/port.txt. Now writes to .dfx/replica-configuration/replica-1.port (same as dfx start) and updates signals.bash to reflect this.
Moves some common actor start logic out of commands/replica.rs and commands/start.rs to actors/mod.rs

Todo:
add ic-ref variant in signals.bash test
refactor common logic in dfx bootstrap and dfx start to actors/mod.rs

[p-shahi]

existing test passed https://hydra.dfinity.systems/jobset/dfinity-ci-build/sdk.pr-1349