accounts/external, internal/ethapi: fixes for London tx signing (#23274)

Ticket #23273 found a flaw where we were unable to sign legacy-transactions
using the external signer, even if we're still on non-london network. That's
fixed in this PR.

Additionally, I found that even when supplying all parameters, it was impossible
to sign a london-transaction on an unsynched node. It's a pretty common usecase
that someone wants to sign a transaction using an unsynced 'vanilla' node,
providing all necessary data. Our setDefaults, however, insisted on checking the
current block against the config. This PR therefore adds a case, so that if both
MaxPriorityFeePerGas and MaxFeePerGas are provided, we accept them as given.

OBS This PR fixes a regression -- on current master, we are unable to sign a
london-transaction unless the node is synched, which may break scenarios where
geth (or clef) is used as a cold wallet.

Fixes #23273