Update withdrawal params for two step withdrawals

[trianglesphere]

Description

This renames the L2 block field to the L2 Index. This more accurately matches what the L2 output oracle contract expects.

Tests

Tested in hive here: ethereum-optimism/hive#66

Additional context

I think we need a design doc on the API design of the proof withdrawal utilities. I expect that the API might change again - having to pass in the contract is a little bit ugly.

Once this is merged, I'll have to cut a release v0.10.3 and then update hive with that release.

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[tynes]

I think we need a design doc on the API design of the proof withdrawal utilities. I expect that the API might change again - having to pass in the contract is a little bit ugly.

I agree with this, the sdk has a stable interface for typescript and the existing go utils aren't working well for my usecase of submitting migrated withdrawals