BuildSpec subcommand for Parachains

[shunsukew]

This PR is for feature request.

Substrate CLI has BuildSpec subcommand (https://github.com/paritytech/substrate/blob/master/client/cli/src/commands/build_spec_cmd.rs), however, it cannot generate ChainSpec with user-specified Parachain ID.

BuildSpec subcommand for Parachain specific cases and allowing users to generate ChainSpec with arbitrary Parachain ID would be good to have, because the more XCM stuff coming, the more demands to configure and run multiple chains flexibly (even they are same runtime chains) with different Parachain IDs in the same Relaychain.

This is doable by having BuildSpec in cumulus and take --parachain-id as an optional flag.

[bkchr]

The --parachain-id flag will not return to Cumulus. See some justification here: #739