feat: add `Permit2` approval process for `UniversalRouter`

[0xAlec]

What changed? Why?

• add Permit2 approval flow for V2 API
• in our V2 API, we use Uniswap's UniversalRouter
• this is different than the standard approval process, where you approve ERC-20 tokens for spending against the router directly
• instead, for UniversalRouter ERC-20s are approved against an intermediary Permit2 contract
• this adds an additional step in our approval flow, since we need to approve the UniversalRouter contract to spend the funds approved against Permit2
  • this is typically done as a (gasless) signature - but we implement this as a transaction to allow for batching by smart wallets
• blockchain security recommended us only approve exact amounts to Permit2 (typically the approval is unlimited and only done once - because of the this constraint we need a transaction for each Permit2 approval, making the swap experience 3 separate transactions for EOAs)

read more: https://blog.uniswap.org/permit2-and-universal-router

Notes to reviewers

How has it been tested? using https://github.com/ilikesymmetry/onchainkit-demo

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