feat: decode multisend payload and add calls from it

[banteg]

What I did

add MultiSend.add_from_calldata that decodes a multisend payload and adds it as calls.

i've also added a test to demonstrate it encodes back to the same calldata.

How I did it

im treating the data as a buffer since the data encoding uses a length prefix. when the buffer is exhausted, we have decoded all calls.

How to verify it

read the encoding spec here https://etherscan.io/address/0x40A2aCCbd92BCA938b02010E17A5b8929b49130D#code

Checklist

• Passes all linting checks (pre-commit and CI jobs)
• New test cases have been added and are passing
• Documentation has been updated
• PR title follows Conventional Commit standard (will be automatically included in the changelog)