Add apify env pull #1573

apify env pull writes the Actor version's variables into .env, so a local project can start from what the platform has.

Scope

• Write plain variables with their values. Write secret variables with an empty value, because the API never returns them.
• When .env exists, update plain variables, keep the local value of keys that are secret on the platform, and keep keys that exist only locally. Show what changes and ask for confirmation. --yes skips the prompt.
• Add .env to .gitignore when it's missing.
• List the written keys without values when --json is set.

Done when

• Tests cover a new .env, a merge with an existing .env, and a version with secrets.

Depends on

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