sdk: Implement borsh traits via a proxy crate

[joncinque]

This is a copy of solana-labs#32668

Problem

solana-labs#32511 adds the borsh 0.9 traits to Pubkey, but because of how the crate name is resolved in the macro, the implementations for the older traits have to be done by hand, which is error-prone and tedious.

Proposed Solution

Implement the traits via a proxy crate, just like https://github.com/ilya-bobyr/borsh-multi-version/