[proxyd/op-txproxy] multicall support as a library

[hamdiallam]

Is your feature request related to a problem? Please describe.
op-txproxy can leverage proxyd to multicall to n+1 sequencer backends. While this is good for us as we use proxyd, it doesn't feel right to require proxyd as it's supplemented infra in the op-stack

Describe the solution you'd like
In order to remove the need for an extra service, leverage proxyd multicall routing strategy as a library instead as an external service

Will be exploring this route, maybe there's some good rational to enforce that every user of op-txproxy must use proxyd