fix: credit origin chain ID on SignedFills

[anna-carroll]

Problem

Within Fills, the Output chain ID is intended to indicate the origin chain ID, NOT the destination chain ID.

Confusion arose because the Solidity was designed to "overload" the chainId field on the Output struct depending on the context; in the context of Orders, it should specify the destination chain ID; in the context of Fills, it should specify the origin chain ID.

Previously, the SDK Fill signing logic incorrectly populated Outputs with the destination chain ID.

Fix

Fill Outputs must specify the origin chain ID.

Approach

This PR updates the Fill signing logic in the UnsignedFill struct to explicitly add the origin chain ID before signing.

Alternate Approach

If, at some point in the future, AggregateOrders was used to agg Orders from different origin chains, this logic could become problematic. It may be prudent to address this one step sooner within the AggregateOrders type instead -- by specifying a single origin chain ID for a given AggregateOrders, or else by splitting Orders up within AggregateOrders by respective origin chain IDs. However, this change is more involved.

Open to discussion on which approach is preferred.

[prestwich]

i don't think the name origin is very descriptive here. i think that the concept we're trying to communicate is what chain the fill is for. i.e. "this can only be used by the chain with id X" the same way that all the other structs/events specify the rollup chain id