Add apify env diff to check that .env and the platform are in sync #1574

A check that fits pre-commit hooks and CI: does the platform have the variables the local project expects?

Scope

• Compare .env with the Actor version. List names that are only in .env, names that are only on the platform, and plain variables whose values differ.
• Match secrets by name only. Their values can't be compared.
• Exit with 0 when both sides are in sync and with 1 on any difference.
• Support --json. Never print a secret value.

Done when

• Tests cover the three kinds of difference and the exit codes.

Depends on

• The apify env namespace and targeting (Add the apify env namespace and apify env ls #1570)