Add eth_fillTransaction

[s1na]

This helps users fill in transaction fields when the signing is supposed to take place outside the node. This has been implemented in Geth since EoY 2019 but unfortunately was never proposed for inclusion in the specs.

The input to the method is a GenericTransaction object. The response in the following form:

{
  "raw": "0x...", // RLP-encoded tx
  "tx": {
      ..., // filled in tx fields
  }
}