[format] Integrate elastic/celfmt to format CEL programs

Integrate https://github.com/elastic/celfmt into the `elastic-package format` command to apply canonical format to CEL programs contained in `cel.yml.hbs` files associated with `input: cel`. File names associated with CEL input types should be determined through the `template_path` value.

Formatting the content consistently helps with the review process.

### `template_path` specification

The `template_path` can occur in two locations. And I believe the undocumented default is `stream.yml.hbs`[^1].

- https://github.com/elastic/package-spec/blob/507622633e4ee5974bb0f07a87e51141839f6e3c/spec/integration/data_stream/manifest.spec.yml#L543

- https://github.com/elastic/package-spec/blob/507622633e4ee5974bb0f07a87e51141839f6e3c/spec/integration/manifest.spec.yml#L504-L508

[^1]: https://github.com/elastic/kibana/blob/ed5f3e714cca887ff88f1f6cd6273a94a9c89e66/x-pack/platform/plugins/shared/fleet/server/services/epm/archive/parse.ts#L506

Start at the `elastic-package format` command and inspect how `template_path` is defined in the linked package-spec locations. Review the `elastic/celfmt` integration requirements and determine how CEL input files in `cel.yml.hbs` are identified. Done means the command applies canonical formatting to those CEL programs using their configured template paths.

Written by the indexing model from the issue text.