Filter azd app env output by variable prefix

[jongio]

Large services often resolve dozens of environment variables, but users usually need one group such as AZURE_, DATABASE_, or SERVICE_. Prefix filtering would make terminal output easier to scan without exposing unrelated values.

Acceptance criteria:

• azd app env <service> --prefix AZURE_ prints only matching variables.
• Multiple --prefix values are allowed and match if any prefix applies.
• The filter works with dotenv, shell, json, --keys, and --all.
• The command explains when no variables match.