Multisig: Implement the indexer

[Braqzen]

Indexer Guideline

There should be a single wasm module which contains multiple functions, 1 per event. The fn name may follow the pattern index_<event_name>.

E.g.

#[indexer(manifest = "<path/to/file/multisig.manifest.yaml>")]
mod multisig_index {
    fn index_cancel_event(event: CancelEvent, block: BlockData) { ... }

    fn index_execute_event(event: ExecutedEvent, block: BlockData) { ... }

    // more events below
}

Each type in graphql should have a required field.

Certain event fields would benefit from the @indexed directive.

• CancelEvent
  • user
• ExecutedEvent
  • to
  • value
• SetWeightEvent
  • user
• TransferEvent
  • asset_id
  • to
  • value

From the BlockData we should retrieve the block height and add it to each event so that we have a measure of time for when the event took place.

[bitzoic]

Closing in favor of #605