chore: Add support for buy orders with same sell and buy token

[m-sz]

Background

The PR #3894 introduces sell=buy token feature, currently supporting only sell orders, being the first milestone. Buy orders seem to be more complicated regarding quoting and verification so they have been left out as the next step.

Buy orders for sell=buy are required for programmatic orders on CoW Swap:
General idea:

• buy exactly 1000 A for at most 1010 A
• recipient: Cow-shed (special user control SC with 1271 support)
• posthook: Setup the TWAP for user's cow-shed
  more details: Implement composable cow for cow-shed cowdao-grants/cow-shed#53

Detail

Figure out proper solving, quoting and verification solution for BUY orders in case of sell=buy token.

Acceptance criteria

A list of tasks that need to be done for the issue to be considered resolved.

[squadgazzz]

Was it confirmed somewhere that this feature is even required for buy orders? Any specific reason someone would prefer buy orders over sell ones? Please update the description.