[feat req] Merge artifacts option

[mmuzikar]

What would you like to be added?

An option to not throw 409 Conflict when an existing artifact exists, but merge the artifacts as it was possible in the previous version.

Why is this needed?

In v3, it was possible to have an artifact of the same name that would get merged when a new upload action was invoked, and as a result, it was really easy to create an aggregate artifact with the results of all matrix jobs. As an example, I had a test-results artifact that contained folders for each matrix combination containing all the matrix runs. This was really nice for reporting summaries for PRs, and collecting the results in one click.