pop build parachain: Generate a chain specification file

[AlexD10S]

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

[weezy20]

Comment on the use of the word/subsubcommand parachain. Does this need to be under build more specificially, build parachain? Where possible I recommend designing at most, 1 subcommand deep commands to emphasize the quickness working with pop-cli

Here's some design alternatives to consider:

1. pop build-spec <options> where options are simply piped to build-spec on the substrate cli.
2. pop build spec <options> again, this means a separate subsubcommand but atleast the intent is clearly communciated with it and not using parachain everywhere redundantly. This is also important for future considerations where users will be allowed to use pop with custom templates/repositories (pop new --url ) which would mean avoiding code modification when we want to build-spec out of a solochain codebase.
3. pop export <chainspec | runtime-wasm | genesis > a new export subcommand that deals specifically with exporting artifacts that are useful to the parachain developer.