Permit Signatures - Document relevant info

[brunoguerios]

Relevant info to be shared regarding using Permit and Permit2 signatures in the SDK:

• Balancer v3 allows token approval flow to be handled through Permit and Permit2.
• It can be done through direct approvals (ERC20 or Permit2) or through signatures.
• To facilitate using signatures, SDK:
  • includes buildCallWithPermit that expects Permit signatures for Remove Liquidity operations
  • includes buildCallWithPermit2 that expects Permit2 signatures for Add Liquidity and Swap operations
  • provides helpers to sign Permit and Permit2 signatures
• all permit signatures operations (swaps, add/remove liquidity) support native asset as input/output (i..e ETH)

[johngrantuk]

To clarify final "docs":

• some content to readme (including eventual limitations, such as does not support wethIsEth = true)
• examples - I don't think we need one for each case or anything but maybe some inline comments explaining whats happening and why
• docs site - I think its important to add this info to the docs site somewhere, maybe even its own section