feat: added Optimism deposited transaction support

[merklefruit]

Motivation

Optimism has an extra transaction type: the Deposited transaction (type 0x7E), with 3 new transaction fields as described in the spec here. The RLP encoding order is also different.

Solution

This PR introduces a new feature "optimism" much like the existing "celo" one, which enables the 3 new optional transaction fields.

PR Checklist

• Added Tests
• Added Documentation

[prestwich]

I'm supportive of adding first-class optimism objects. However, I am concerned that this change might be too large for our current release cycle. We're trying to put this version into maintenance so that we can focus on ethers@3.

In addition, our opinion generally is that the Celo feature was a bad approach to network-specific behavior in the first place, and this ought to be solved by abstraction of the argument and return types in the Middleware trait

@gakonst do you think it is worth it to unblock optimism folks?

[gakonst]

I would like to add this because it also unblocks op-reth work for client side. I am open to merging this if CI passes

[prestwich]

Tests need to be fixed

[prestwich]

Need to update CI to run with the optimsim feature enabled.
Need to document that Optimism and Celo are mutually exclusive

[merklefruit]

Ok, I've split the addition of a new TypedTransaction variant to a secondary PR to make the diffs cleaner.
Here: merklefruit#1

I've also added a couple tests for RLP encoding/decoding of this new tx type. Let me know what other kinds of tests I should eventually add!

[merklefruit]

@gakonst I believe the PR above (merklefruit#1) was not merged into this branch yet. I've closed it now, so I need to reopen a PR from this branch