Support using custom metadata in index announcements

[hannahhoward]

Goals

Provide an option that allows using custom metadata in index provider announcements

Implementation

• add a storage provider option that allows passing a metadata data generation function that given a deal, generates metadata for the deal
• by default, just use a function that generates graphsync metadata (change is backwards compatible unless config option is used)
• in AnnounceAllDealsToIndexer, skip provider.ErrAlreadyAdvertised -- this isn't really an error in this case, just a no op skip

PR supports Bitswap content indexing in Boost -- PR to come in that repo