Introduce ensure-only-one: name to example codestarts

[ia3andy]

Description
Until we resolve this at extension level, we need to make sure that some example codestarts of different implementation of a feature (that would collide if used together) is handled

This issue was raised by @patriot1burke in #11243

Implementation ideas

We could add a new field in codestart.yml:

ensure-only-one: funqy

If more than one funqy is selected, then generation will fail with explanations.

@maxandersen could you link the extension discussion about this?

[quarkusbot]

/cc @quarkusio/devtools

[ia3andy]

We now support this using the singleton tag: https://github.com/quarkusio/quarkus/blob/master/devtools/platform-descriptor-json/src/main/resources/codestarts/quarkus/singleton-examples/amazon-lambda-example/codestart.yml#L6