Get signer of the transaction

This issue blocks #22

### The problem:

if you try to make this query [here](https://graphql-console.subsquid.io/?graphql_api=https://moonriver.archive.subsquid.io/graphql)

```graphql
query MyQuery {
batch(
limit: 10
fromBlock: 569006
toBlock: 569006
ethereumTransactions: {
contract: "*"
data: { call: { origin: true, _all: true } }
}
evmLogs: { contract: "*" }
) {
header {
height
}
extrinsics
}
}
```

You will see that `signature` field on extrinsics array is null.

It means that we can get who was the `caller` of the tx

### Workaround:

> here is an example of serializing and parsing substrate eth transaction data with ethers
https://github.com/belopash/squid/blob/ccc1ebb0308d041ea07d7376f369a36b6430ada3/test/astar-erc20/src/processor.ts#L24

Start with the batch GraphQL query in the issue and the linked test/astar-erc20/src/processor.ts workaround for serializing and parsing Substrate Ethereum transaction data. Trace how extrinsics are exposed, then verify the query returns a non-null signature or signer for the affected transaction.

Written by the indexing model from the issue text.