[FEATURE] Parse Array of flags Objects From All Sync Sources

Background

flagd needs to support flags defined in an array, as well as the previous map format.

Requirements

• Flags must be parsed, and stored in memory, annotated with both their flagSetId (nil is a possible flagSetId) as well as their source
• Refactors to the underlying storage layers are encouraged (this was mostly done with (chore(refactor): use memdb for flag storage #1697).
• Particular care should be taken to make retreival of sets of flags based on their flag set efficient and easy
• Associated tests

See parent issue for more background.