Support multiple components in changelog renderer template

[endorama]

#55 renderer implementation does not support multiple component, and the value is hardcoded to {agent}.

The renderer should rely on the component field of the fragment instead (by grouping entries kind and then by component).

Reference template: https://github.com/elastic/observability-docs/blob/5e2f16b351769a2d9ed7373d5e3290c16498eefa/docs/en/ingest-management/release-notes/release-notes-8.2.asciidoc

[endorama]

We should be able to configure a map with valid components and their "human readable" counterparts.

Components should not be a free form field.

[endorama]

We reject any fragment whose component is not in the list of valid components. If the list is empty we force the component field in fragment to be empty as well.