Multisig operations. Use indexer for getting multisig operations

[pgolovkin]

The problem

There are 3 problems with current approach for getting and decoding multisig operations:

1. Multisig operations that are executed or rejected won't be added into the operations list because they are removed from multisig pallet storage.
2. Multisig operations call data may be retrieved only from the archive node if it was created more then 30 blocks ago (~180 seconds on Polkadot)
3. If runtime upgrade happened before the user gets the multisig operation it may not be decoded.

Proposed solution

Use indexer for getting operation list.
Pros:

1. The user will get all multisig operations including executed and rejected
2. The user will get correct block number when the operation was executed, rejected, signed
3. The call data and decoded call data will be always provided for exectued transaction
4. The call data will be always provided for ongoing and rejected operations if call data was sent to blockchain
5. The call data decoding will be decoupled from the runtime update
6. If multisig initiated with batch, proxy call it will be decoded
   Cons:
7. Indexer may not be available
8. Indexer may be attacked and the user will get wrong decoded operation information

How to avoid security risks

1. Compare call hash and hash(call data)
2. For ongoing operations check if operation with call hash exists on the blockchain in multisig pallet storage