Abstract out AppendVec into AccountsFile enum

[yhchiang-sol]

Summary of Changes

This PR abstracts out AppendVec into AccountsFile enum.
This will allow different implementations of AccountsFile that support the AccountsDB.

[brooksprumo]

Some musings on the name. Cause it'll only be harder to change going forward 😄

[brooksprumo]

Wondering about the impl side of this enum. Is an enum the right abstraction here? Maybe a trait makes more sense, and the AppendVec and the new ColdStorage both implement that trait?

[yhchiang-sol]

Wondering about the impl side of this enum. Is an enum the right abstraction here? Maybe a trait makes more sense, and the AppendVec and the new ColdStorage both implement that trait?

The implementation of the new storage using this enum is in #28790 (I haven't rebased yet so it's still using the old name AccountsFileEntry.)

I think we can keep the enum for now to maximize the flexibility. Once we've finalized the eventual API together with the cache and in-memory accounts representation, we can revisit here whether we want to keep the enum or do a trait instead. Converting a trait should be fast, but I would like to focus on the implementation for now, but without merging this PR I will need to constantly fix merge conflict 😂.

[brooksprumo]

Lgtm