token-2022: Refactor StateWithExtensionsMut functionality into trait

[joncinque]

Problem

As part of moving token-2022 to Pod-style deserialization in #6316, we'll eventually need to make new PodStateWithExtensions*, in which the BaseState itself is a Pod type.

That's easy for the non-mutable case, since we have BaseStateWithExtensions that neatly encompasses all of the functionality on a trait. We don't have a mutable version of BaseStateWithExtensions.

Solution

Create BaseStateWithExtensionsMut. This essentially moves all of the common functionality of StateWithExtensionsMut into a trait requiring only two functions:

• get_tlv_data_mut: just the mutable verison of get_tlv_data in BaseStateWithExtensions
• get_account_type_mut: some way to access the account type part of the slice in a mutable way, so that it can be written to during initialization

This PR contains no functional changes, just moving functionality to the new trait.

[CriesofCarrots]

One small question, one suggestion for deduping