Allow to skip --parachain-id

[h4x3rotab]

In some situation, we already have the parachain id embedded in the chain spec file, and thus --parachain-id is not used (and maybe is even not supported in some node implementation).

This PR introduces "skip_id_arg" option in ParachainConfig to deal with that.

[shawntabrizi]

@h4x3rotab sorry i think this is a little icky...

Can we do something like: if the id is missing, and there is nothing hard coded, then throw an error.

Otherwise, if id is missing, use what is embedded.

If id is specified, use that.

[h4x3rotab]

Thanks for the feedback. Do you think we can make it this way:

1. If no parachain-id is specified in the config file, we try to read it from the chain spec (we can build the chain spec and read the id from it anyway)
2. When parachain-id is not specified, we don't pass --parachain-id flag to the node on startup

For some node implementations where --parachain-id is always required, it just throw an error at launch, warning the user directly

[shawntabrizi]

@h4x3rotab yeah something like that seems fine.

Basically i do not want to introduce a new flag saying to skip some other flag. I want that behavior to be determined implicitly from either including or excluding the id argument.

[h4x3rotab]

@shawntabrizi Fixed as you suggested and rebased to the latest commit

[h4x3rotab]

Kindly ping