[MNG-7560] Provide a standard way to access the PGP signature for an artifact

**[Christoph Läubrich](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=laeubi)** opened **[MNG-7560](https://issues.apache.org/jira/browse/MNG-7560?redirect=false)** and commented

I recently needed to access the PGP signature of an artifact but found two problems:

1. It was surprisingly hard to get hold of it, I ended up in creating another artifact request for a special type so maven downloads it
2. I instantly was presented with a warning like described here: https://github.com/s4u/pgpverify-maven-plugin/issues/53

It would be good if maven would contain a method (e.g. in the ArtifactSystem or somwhere else) that allows to get the signature of an artifact, or some option when downloading an artifact that says what additional checksum (sha512 for example) or signatures (asc for example) should be made available alongside in the local repository.

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No further details from [MNG-7560](https://issues.apache.org/jira/browse/MNG-7560?redirect=false)