feat(balances): Eliminate requirement for balance fetcher

[immasandwich]

Description

• Instead of a single BalanceFetcher class, Studio will allow you to register a balance fetcher class per app token group and contract position group.
• For app token groups, the default strategy is to call erc20.balanceOf on all of the indexed tokens
• For contract position groups, the default strategy is to return an empty array
• Users can register a balance fetcher per group to override the default strategy (generally required for contract position groups because of their non-standard nature; generally not required for app token groups because of the ERC20 standard)
• Enable a registering a presenter to re-group balances and hydrate the response with additional metadata
• The default presenter will simply group balances by their groups in the app definition and present them

Checklist

• I have followed the Contributing Guidelines

How to test?

http://localhost:5001/apps/rari-fuse/balances?addresses[]=0x9c5083dd4838e120dbeac44c052179692aa5dac5 (Rari Fuse balances for pennilesswassie.sismo.eth; regrouped and hydrated with metadata)
http://localhost:5001/apps/aelin/balances?addresses[]=0xde6d499de897c1c989e2bc0b3fce48626c84a6c3 (Aelin balances for gaojianxiao.eth; default afterware)