feat: add basic policy middleware

[mattsse]

Motivation

Support a way to enforce certain rules for transactions

Solution

Add a new PolicyMiddleware type which wrapps a middleware and includes an instance of the new Policy trait, which is a single function to ensure a transaction is in compliance with this policy.

Further work

common use cases could be to look up the receiver in a database and check if its address is whitelisted. Not sure yet what the most flexible implementation would be since a DB adapter is database specific and also how to check for a whitelisted address may be database model specific. So we could abstract basic use cases via some kind of PolicyBackend trait or smth which implements common functions for checking is_whitelist(&self, recv: Address) and also for checking the calldata maybe. We could also add a default persistent implementation of that (postgres for example) and a HashSet based one and feature gate this.

[gakonst]

Excellent, this interface looks great: async fn ensure_can_send(&self, tx: TypedTransaction) -> Result<TypedTransaction, Self::Error>;

I don't think we really need the policy backend trait given the quirks of each individual platform.

Let's start with a simple in-memory one, and we can start adding databases afterwards. Agreed that if they introduce new deps (eg tokio-postgres) they should be feature gated.

Simple functionalities:

• to can only be in whitelist
• from can only be in whitelist
• value cannot be over X ETH
• data can only follow certain patterns (e.g. match first 8 bytes to limit function selector, or even more granularly limited the args based on the function selector)