Support the shared store in apify env #1580

The apify env commands manage the store and the references to it.

Scope

• Add --store to apify env ls, set, and rm. With it, the command manages the account's shared variables instead of the Actor's. The flag is named --store because a "global" flag can be read as either per Actor or per organization.
• Add apify env link <stored name> [--as <name>], which makes the Actor version reference a shared variable. apify env rm <name> removes the reference.
• Mark references in apify env ls. In apify env pull, write the value of plain shared variables and an empty value for shared secrets. In apify env diff, treat a reference like any other variable.
• Never replace a reference with a literal value in apify env push without showing it in the diff.

Depends on

• A release of the JavaScript client with shared env vars (Support shared env vars in the JavaScript client apify-client-js#1209)
• apify env push (Add apify env push #1572)
• apify env pull (Add apify env pull #1573)
• apify env diff (Add apify env diff to check that .env and the platform are in sync #1574)