Add config option to get block number from RPC instead of the contract

[Siegrift]

Related to #273

On some chains block.number on chain does not work as expected and we need to call the getBlock from RPC instead. While this should be fixed properly in the issue above, there are more chains with this property and it makes sense to add a config option for these (instead of hardcoding these chains in the code).

Once #273 is implemented, the config option should be removed.

[Siegrift]

As a note, Burak had a good point on the call, that having this as config option is nice even if there is some flag in @api3/chains, because it allows running Airseeker with chains that are not released in @api3/chains.

So in general, the airseeker config creator should be based on @api3/chains, but generate the Airseeker config and utilize its config option for this. Airseeker should not depend on @api3/chains.

[Siegrift]

This is no longer needed. See #314 (comment)