Add Windows cmd output to azd app env

[jongio]

Windows users often need environment output that can be pasted into cmd.exe or saved as a batch file. Today they must translate dotenv or shell output by hand.

Acceptance criteria:

• azd app env <service> --format cmd prints set KEY=value lines.
• --all groups services the same way as the text formats.
• Masking still protects secret-shaped values by default.
• Tests cover command output and unsupported format errors.