Add apify env push #1572

apify env push uploads .env to the Actor version. It replaces the implicit upload that apify push does today.

Scope

• Read .env from the Actor directory and compare it with the variables on the Actor version.
• Show a diff (added, changed, unchanged, and with --prune removed) and ask for confirmation. --yes skips the prompt. Without a terminal and without --yes, fail.
• Add and update only by default. With --prune, also delete platform variables that aren't in .env.
• Keep the type of an existing variable. Create a new variable as a secret, or as plain when --plain is passed.
• Mark an existing secret as replaced in the diff without comparing values. Secret values can't be read back.
• Skip a key with an empty value and say so. This keeps apify env pull followed by apify env push from blanking secrets.
• Use the per-variable API calls in the first version, and report the result for each variable if a call fails midway.
• Print the diff and the result without values when --json is set.

Done when

• API tests cover add, update, --prune, empty values, and the non-interactive path.
• No value appears in output, logs, or telemetry.

Depends on

• The apify env namespace and targeting (Add the apify env namespace and apify env ls #1570)
• .env is never uploaded (Never upload .env, and ignore it in Git by default #1568)