Propose a parent pom for external extensions

[danielpetisme]

Currently, the extensions are embedded into the main Quarkus repository thus use the internal build-parent pom.xml.
To permits 3rd parties to develop their own extensions, it would be great to propose a public quarkus-extension-parent pom.

https://quarkusio.zulipchat.com/#narrow/stream/187030-users/topic/Extensions/near/161188290

[ppalaga]

I proposed a basic parent for third party extensions in #3427 the commit: ppalaga@5120227

However, when we start refactoring the hierarchy, @aloubyansky would like to solve more than this single problem #3427 (comment)

This is basically the same approach I was investigating. I was just thinking it could be organized slightly differently, e.g.

build-parent/
|- application/
|  `- pom.xml (parent for apps built on Quarkus core, flattened)
|- extension/
|  `- pom.xml (parent for extensions built on Quarkus core, probably has application/pom.xml as its parent, flattened)
`- internal/
   `- pom.xml (Quarkus core build parent pom, has extension/pom.xml as its parent).

However, before actually applying this kind of change, it'd be a good idea to announce/discuss it on the dev list.

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[danielpetisme]

Fixed by https://github.com/quarkusio/quarkus-platform fix