Implement node version

[Siegrift]

Closes #82

Rationale

Added nodeSettings.nodeVersion similar to the Airnode. I am not entirely happy with the naming, but sticked to the on used in Airnode. Also, I put the node version inside nodeSettings in anticipation for other fields (e.g. stage).

I also noticed that I didn't completely remove source maps in #64 so I removed the (no longer necessary) --enable-source-maps flag.

[bbenligiray]

In the Airnode config airnodeWalletMnemonic is also under nodeSettings https://github.com/api3dao/airnode/blob/master/packages/airnode-node/config/config.example.json#L68
rateLimiting only exists in Airseeker and it's at the config root for the lack of a nodeSettings field (https://github.com/api3dao/airseeker/blob/main/config/airseeker.example.json#L3), but I'd argue that if there is a nodeSettings field, rateLimiting should go under it