ByronSpec integration: don't take SoftwareVersion/ProtocolVersion/key from genesis/ledger

At the moment, the ByronSpec integration

* chooses the protocol version from the ledger state during block production (`forgeByronSpecBlock`),
* chooses a key from the initial genesis delegates in setup (`protocolInfoDualByron`)

Instead, we should be able to "boot up a ByronSpec node" with a _delegated_ key, and with _different_ protocol/software version. This will be essential if we want to test block production _with_ update proposals/votes and delegation certificates. When those are posted, nodes would need to be (emulated to be) restarted with new keys/software versions in order to make progress.