Enable getting getL2Network via an rpc url

[roninjin10]

A general pattern I think all rollups should land on as we move to a multi rollup world is the ability to get the rollup config on chain. This way you can bootstrap the arbitrum sdk with only the rpc url even if the rollup config hasn't been added to this sdk yet.

e.g.

const l2Network = await getL2NetworkFromRpc(
  l2RpcUrl /** <-- rpc url of target Arbitrum chain */
)

This allows us to more easily make tooling work for all chains without needing to specify the config within the library first or needing to tediously craft your own config

[roninjin10]

The way we are thinking about this at OP is sticking the config at a predeployed address on l2. That way any config can bet fetched from a known address