Reimplement sell=buy PoC PR

[m-sz]

Sell=buy token initiative

The ability to swap the same token pairs unlocks the potential for CoW to generally become a transaction relay service. This feature will allow users to execute pre- and post- interaction hooks as gasless transactions. The overall implementation has certain caveats:

• Each settlement has to go through the settlement contract anyway, so the sell=buy token transactions would require users to send X amount of sell token and receive X - cost of the token back. It is not realistic, with the current state of GPv2 contract for the user to just send the cost amount and save on gas this way.
• The solvers can really compete only by batching to save on gas costs
• There is little incentive for this feature to be supported by other solvers, but having this support in baseline solver could generally be enough.

Lift restrictions on sell=buy token

Based the changes on https://github.com/cowprotocol/services/pull/2513/files, adapted to current state of the codebase.

• Adds a command-line argument to the orderbook API to allow for same sell and buy token swaps. The SameBuyAndSell token error is not raised in that case
• Adds e2e test case that checks if sell=buy quoting and orders are possible
• Liquidity fetching happens only if the sell and buy tokens are different
• Relaxes the invariant on Tokens struct to allow for same token pairs
• Minor tweak to Justfile to allow for custom filtering and extra args to nextest

Based on input from @fhenneke:

• Adds a short-circuit behavior for solver to return empty route in case sell=buy
• Returns sell and buy assets directly when solving for empty route

[fafk]

Not related to your changes, but it's super confusing that the method is called liquidity_pairs (plural) when it only returns one pair. 😵‍💫

[m-sz]

I will figure out a more fitting name

[fafk]

What are these asserts for? 🤔 What does it signify that the order has a verified quote and has metadata?

[m-sz]

These assertions are also present on the normal place_order_with_quote and it makes sure that the quote and order's quote are verified.

[squadgazzz]

I think this PR needs to be converted to a draft, since more changes are expected. @m-sz , is that correct?

[m-sz]

Yep, had a bunch of 1:1s and will be adding feature flag to the whole change to control it in orderbook.

[m-sz]

I have updated the PR with CLI argument to orderbook to allow for same sell and buy tokens.
Changes to baseline solver, to support those transactions will come in a separate PR.