Adds inspect_account() to TransactionProcessingCallback

[brooksprumo]

Problem

For the upcoming accounts accumulator hash, we will need a way to get the initial state of an account before transactions update it.

A generic way to inspect accounts prior to transaction processing would solve this.

Summary of Changes

Add a new method to TransactionProcessingCallback, inspect_account(), that will provide a callback for the loaded accounts in a transaction.

[brooksprumo]

@HaoranYi Can you take a look at this PR to ensure it'll work for our lattice hash needs? Assuming it works for us, I'll add some additional svm experts to review as well.

Here's an example of me using this new method: https://github.com/brooksprumo/solana/blob/755ccd379e5ad3a2ee241406feae673963277e18/runtime/src/bank.rs#L5220-L5253

[HaoranYi]

looks correct to me. And it seems to be a useful interface to have!

[buffalojoec]

Lgtm, just a suggestion on the variant names!