l2geth: bring back unsupported RPC methods

Previously, any RPC methods involving hot keys and
signing by the node were disabled when the node
ran with the OVM configured. This was to prevent
users from attempting to use these methods against
the live sequencer. Now that the infrastructure is
more mature with `proxyd`, particular RPC requests
can be routed appropriately and blocked at the infra
level. Allowing these methods makes local development
easier.

This change cannot be adopted without coordination
from infra providers, ensuring that they will block
these methods at their infrastructure. Each optimism
node has at least 1 key that is used to ensure
block production is deterministic when running in
clique mode.

Also forward transactions to the sequencer when
running as the verifier for the RPC endpoint
`eth_sendTransaction`. This RPC utilizes a key
that the node is managing.