Add apify env set and apify env rm #1571

Creators and agents need an explicit way to change one platform env var without Apify Console.

Scope

• Make apify env set <name> [--secret] create the variable, or update it when it exists.
• Never take the value as a command-line argument, so it doesn't end up in shell history. In a terminal, prompt for it, masked when --secret is set (useMaskedInput). Without a terminal, read it from stdin. Fail when there is no terminal and no stdin.
• Keep the type of an existing variable unless --secret or --no-secret is passed.
• Make apify env rm <name> delete one variable. Ask for confirmation unless --yes is passed.
• Remind the user in both commands that the Actor needs a rebuild before runs see the change.
• Keep the value out of --json output.

Done when

• A test sets a variable through stdin, sees it in apify env ls, and removes it.
• The value never appears in output, logs, or telemetry.

Depends on

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