Validate env vars against the schema in the CLI #1581

The same check before code leaves the machine.

Scope

• Validate .env against the schema in apify run before the run starts, and list the missing or invalid variables.
• Report required variables that are missing on the platform in apify env push and apify env diff.
• Add required keys that are missing, with empty values, in apify env pull.

Depends on

• The schema definition and validator (Add the env var schema definition and validator apify-shared-js#734)
• apify env diff (Add apify env diff to check that .env and the platform are in sync #1574)