Align `Balance` schema, protocol `Resource` example, and HTTP response example

[kirahsapong]

The Balance resource is demonstrating a couple variations throughout the spec, prob good to align these.

The variations are:

• { data: [ { metadata, data: { balances: [ balanceDetail0, balanceDetail1, balanceDetail2, ... ] }, signature } ] } (Balance as an array within a Resource)
• { data: [ balanceDetail0, balanceDetail1, balanceDetail2, ... ] } (Balance as the entire resource, but not a Resource)

More info here:

• https://github.com/TBD54566975/tbdex/pull/221/files#r1532988686
• https://github.com/TBD54566975/tbdex/pull/221/files#r1533031510