Add `env --format github-actions` for GITHUB_ENV output

[jongio]

azd app env can print a service's resolved environment as dotenv, shell, or json. Running services in GitHub Actions needs a different shape: values appended to the $GITHUB_ENV file, with multiline values wrapped in heredoc syntax so they are parsed correctly.

Add --format github-actions that emits KEY=VALUE lines and, for values containing newlines, the KEY<<DELIM ... DELIM heredoc form that Actions expects. This lets a workflow do azd app env --service api --format github-actions >> "$GITHUB_ENV".

Acceptance criteria:

• env --format github-actions prints single-line values as KEY=VALUE.
• Multiline values use a unique heredoc delimiter that does not collide with the content.
• Output works when appended to $GITHUB_ENV.
• Unit tests cover single-line, multiline, and delimiter-collision handling.
• Documented in the CLI reference.