[RPC-Spec-V2] chainHead: use integer for block index and adjust RuntimeVersion JSON format (#1666)

This PR adjusts the serialized format of the the returned RuntimeVersion
in the rpc-spec-v2 methods. This is done to match the format defined
here:
https://paritytech.github.io/json-rpc-interface-spec/api/chainHead_unstable_follow.html#about-the-runtime

- ##### `apis` field as object
`apis` field of `RuntimeVersion` is now returned as an object, e.g. 
```
"apis": {
      "0xdf6acb689907609b": 3,
      "0x37e397fc7c91f5e4": 1,
}
```
instead of 
```
"apis": [
      ["0xdf6acb689907609b", 3],
      ["0x37e397fc7c91f5e4", 1],
]
```
- ##### removed `stateVersion` and `authoringVersion`
`stateVersion` and `authoringVersion` are no longer returned in the
`RuntimeVersion` JSON Object.

- ##### block index in chain head events as integer

### Related Issues

Closes: #1507
Closes: #1146

### Testing Done
Adjusted existing tests to make sure data is returned in the correct
format.