ADR: user balance in EARN product

[AlexHentschel]

This ADR captures the decision from the Feb 19 Q&A and technical slack discussion.
It specifies how user balances are to be computed in the EARN product.

Key decisions:

• Balance formula to compute user balance denominated in collateral tokens
• Use spot prices (not hypothetical conversions through AMM), consistent with industry convention
• Formula applies to EARN only; ALP does not need converted balances for now
• A single user position in the EARN product (aka "Flow Yield Vaults"?) will only work with a single collateral type for the time being. Users can still utilize EARN with different supported collaterals; they just have to open independent positions, which are independently managed.
• In contrast, ALP design should anticipate multi-asset positions for the future

Please let me know if you think somebody should be added as reviewer. Thanks.