Add parachain related parameters to `chain-spec-builder` (#4889)

When using with `polkadot-parachain`, you usually need to specify the
`relay_chain` and `para_id` fields in the chain spec.

With this PR it can be achieved by specifying newly added `--para-id`
and `--relay-chain` command line args, e.g:
```
chain-spec-builder create -r _runtime.wasm  --para-id 100 --relay-chain xxx default
```

This was implemented by simple _json_ blobs merging.

Additionally unit tests covering basic functionality were added.

Also adds a fix for not overwriting the chain spec with the default
config each time, swallowing not standard fields is also fixed.

Fixes: #4873

---------

Co-authored-by: Sebastian Kunert <[email protected]>
Co-authored-by: Michal Kucharczyk <[email protected]>