Allow matching on any artifact name

[jhrozek]

Changes the artifact ingestor to match any artifact name if the artifact
name is not specified in the rule at all. To allow not specifying the
artifact name, changes the artifact_signature rule_type to no longer
require name in the rule parameters.

[jhrozek]

@lukehinds 👀
To test this, you need to recreate the rule_type:

medic rule_type list --provider=github
medic rule_type delete -I 3 # substitute for a number from the list

and then delete the policy you were using and use one instead that does not specify a name for artifact matching. Here's one I used for testing:

# sample policy for validating artifact signatures
version: v1
type: pipeline-policy
name: jakub-github-policy-artifact
context:
  group: Root Group
  organization: jakubtestorg
  provider: github
artifact:
  - context: github
    rules:
      - type: artifact_signature
        params:
          tags: [main]
        def:
          is_signed: true
          is_verified: true
          is_bundle_verified: true