Create Balances RPC

[apopiak]

The account data (and with it the accounts storage) has changed a few times in the last year which has lead to some confusion for readers (e.g. wallets, exchanges) that did not react to the metadata changes quickly.
It would be useful to have a balances RPC endpoint that returns the balance of a user account independent of the underlying storage (and that changes less frequently).
We basically want to expose similar functions to free_balance via RPC.
Suggested RPC endpoints:

• free_balance
• reserved_balance
• account_data

[xlc]

Is it possible to make those RPC a subscription in order to support subscribe balance changes?

[zachanon]

Hello, I see no one is assigned to this. I would like to hack on this issue, is there any etiquette I must know? Shall I just begin writing tests and code?

[apopiak]

Cool that you want to contribute!
I think it's fine to just start hacking and open a PR. I would suggest opening a draft PR early to get feedback on the general design so you are less likely to paint yourself into a corner.
You're probably aware but there's a contributing guide here.
Feel free to tag me in the PR. Maybe @shawntabrizi and @xlc would also be willing to pitch in.
If you want to go the subscription route (which will make this a bit more complex AFAICT) maybe this PR might help as a reference: #5732