Add package_type column to packages table

What does this MR do?

1. Adds package_type column
2. Set package_type for all current records with maven value.

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/5934

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the [merge request performance guidelines] (https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer
• Reviewed by DB team (@gl-database)