Adding autosubmit configuration pointer file

[ricardoamador]

Adding the pointer file that points to the autosubmit configuration in flutter/.github.

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I listed at least one issue that this PR fixes in the description above.
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[CaseyHillers]

Should this be autosubmit/engine/autosubmit_main.yml?

[ricardoamador]

No. So how it works is that since we use the pointer file we can define a many-to-one relationship with the global files. Really the only one that was different right now is the one for master since flutter has not been moved to that default branch yet. So flutter -> autosubmit/flutter/autosubmit_main.yml and cocoon, engine, packages -> autosubmit/flutter/autosubmit_main.yml since they have the main branch in common.

[ricardoamador]

With that being said I could make it clear by putting the files in specific directories within flutter/.github if that is more readable and easier to follow? I am open to updating the whole config system after I can get the Dart team onboard. Then we can make this things super slick.