Update the archetype schema to match the runtime behaviour

[tvallin]

Fix the schema to document if attribute on exec element

<exec src="foo" if="${bar}"/>

Investigate if if attributes are supported on all invocation directive in the runtime, if so update the schema accordingly.

Publish the updated schema on helidon.io.

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When using the if attribute on exec elements, the schema validation does not fail (version 4.0.11).

Investigate why this does not fail.

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Investigate if the following is supported by the runtime:

<output>
</output>
<exec src="foo"/>

If that works update the schema as well.