chore: add a comprehensive dfx json reference

[sesi200]

Description

Adds dfx schema, a command that prints the current schema for dfx.json. Also added a workflow to keep a copy of the output up to date at docs/dfx-json-schema.json so that the docs can reference the current schema.

First part of SDK-287

How Has This Been Tested?

• manually
• workflow checks that docs/dfx-json-schema.json is up to date
• e2e checks that dfx schema produces valid json

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[smallstepman]

I think building a separate github actions workflow that updates the JSON schema file will suffice, I'm not sure how having e2e test would then be useful. The workflow could run on each push for each PR, and do something along these lines:

if ! diff -q old.json new.json &>/dev/null; then
  git add new.json
  git commit -m "" 
  git push
fi

The workflow could be made :required so it is impossible to merge without completing it first.

[smallstepman]

Suggested change

	pull_request:
	pull_request

the : is unnecessary here

[smallstepman]

more of a fyi, cause I'm not 100% certain here (help @adamspofford-dfinity), but I think this step is unnecessary since we have rust-toolchain.toml file - cargo should be able to use the correct version here

to verify, you might want to try rustc --version instead and see if its 1.60.0

[smallstepman]

ooof chicken and egg problem https://github.com/dfinity/sdk/runs/7451711903?check_suite_focus=true
you probably want to touch docs/dfx-json-schema.json and git add it.

nvm, i see its there