feat(show): add helm show schema cli command

[andser91]

This PR adds a new subcommand to Helm: helm show schema.
Resolves #30726

The helm show schema command displays the contents of a chart's values.schema.json file, if present. It behaves similarly to helm show values: if the schema file is not present, the output is empty (no error).

This feature is useful for users and tooling to programmatically inspect or extract parts of the schema without manually navigating the chart.

• This PR introduces a user-facing changes and should be labeled as docs needed and feature.
• this PR contains unit tests
• this PR has been tested for backwards compatibility

Notes:

• The implementation follows the structure of other helm show subcommands (e.g., values, readme, chart).
• ShowSchema is a new output format handled in pkg/action/show.go.
• cmd/helm/show.go registers the new subcommand schemaSubCmd.
• Unlike other helm show subcommands (such as show values, show readme, etc.), show schema is intentionally not included in helm show all, in order to preserve backward compatibility and avoid unexpected changes in output for users relying on show all.