Add an option for pop build parachain to generate a chain spec file after build it.

The steps are Modify the default chain specification:

1. Generate the plain text chain specification: ./target/release/parachain-template-node build-spec --disable-default-bootnode > plain-parachain-chainspec.json
2. Modify the file to change para_id indicated by the user (flag --para_id)
3. Generate a raw chain specification file from the modified chain specification file: ./target/release/parachain-template-node build-spec --chain plain-parachain-chainspec.json --disable-default-bootnode --raw > raw-parachain-chainspec.json