Implement Cel2 migration tool (#110)

* Add `celo-migrate` tool

* Add logic to write changes to state DB

* Minor changes

* Check that datadir exists

* Use cel2 migration header for rollup config

* Fixes from testing

* Add checks when copying deployed contracts into state db

* Review changes

* Review changes II

* Update op-chain-ops/cmd/celo-migrate/main.go

Co-authored-by: piersy <pierspowlesland@gmail.com>

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Co-authored-by: piersy <pierspowlesland@gmail.com>