Add eth_fillTransaction

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](https://github.com/ethereum/go-ethereum/releases/tag/v1.9.3) 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
}
}
```