Abstract the TEE policy verification logic into a library useable by external contracts

# Problem

[Recently in the FlashblockNumber contract](https://github.com/Uniswap/flashblocks_number_contract/pull/6) we had to copy-paste sections of the BlockBuilderPolicy's caching logic into the FlashblockNumber contract in order for it to be able to outsource verifying the builder role to the BlockBuilderPolicy. This was error-prone and caused duplicative code

# Solution

Refactor this caching logic into a Solidity library that can be easily re-used by external contracts. This will make it so that a policy contract can outsource its auth logic to another policy with as little code duplication as possible.