add accountHealth

[wphan]

add an endpoint for a subaccount's accountHealth (same as health modal on UI).

Should show for both initial and maintenance margin types:

• Total collateral
• Margin requirement
• Free collateral

[jordy25519]

should be simple, can extract this: https://github.com/drift-labs/drift-rs/blob/a443eaf060b2fb54d373a82b5f040a10d3c58099/src/liquidation.rs#L260-L278