This commit adds a SimpleTxManager for tracking and bumping fees on txs
the batch submitter needs to publish. The bulk of the logic is adapated
from the existing tx manager (YNATM) used in the typescript version to
minimize any new classes of bugs that are not already considered.

The manager is configured via a min and max gas price, as well as an
additive gas price step that is applied after each resubmission interval
elapses, before signing and broadcasting a new transaction. This
corresponds to the LINEAR fee policy available in YNATM.

Txs generated from the same call to Send are treated as equivalent, thus
the method blocks until the first tx confirms. Care is taken to
throughly unit test the interactions and edge cases, as subtle bugs in
tx publication can lead to big headaches in prod. To this end, we
achieve 100% test coverage in the txmgr package:

```
coverage: 100.0% of statements
ok  	github.com/ethereum-optimism/go/batch-submitter/txmgr	10.311s
```