fix: issues where was difficult to change network-config defaults in L2 sub-classes

[antazoey]

What I did

L2 network subclasses still wasn't quite for most of the L2s because they changed the default tx.
Now it works great.

For example, this is how BSC looks:

def _create_config() -> NetworkConfig:
    return create_network_config(block_time=3, default_transaction_type=TransactionType.STATIC)


class BSCConfig(BaseEthereumConfig):
    DEFAULT_TRANSACTION_TYPE: ClassVar[int] = TransactionType.STATIC.value
    NETWORKS: ClassVar[Dict[str, Tuple[int, int]]] = NETWORKS
    mainnet: NetworkConfig = _create_config()
    testnet: NetworkConfig = _create_config()

which is a big reduction in custom code needing to maintain

note: l2 plugins can still configure normally, this is only a way to make it simpler for them as an option

How I did it

1. Use class vars DEFAULT_TX_TYPE and NETWORKS
2. Better merging of config data
3. Make local computed so it can utilize the class vars
4. When automatically creating the forked network, use the DEFAULT

How to verify it

Checklist

• All changes are completed
• New test cases have been added
• Documentation has been updated