Add a setting to allow/disallow duplicate package uploads
### tldr
This epic is the home for the issues required to add this new group setting for each supported [package manager format](https://docs.gitlab.com/ee/user/packages/).
### Priority
1. [Maven](https://gitlab.com/gitlab-org/gitlab/-/issues/296895)
1. [Generic](https://gitlab.com/gitlab-org/gitlab/-/issues/293755)
1. [NuGet](https://gitlab.com/gitlab-org/gitlab/-/issues/293748)
1. [PyPI](https://gitlab.com/gitlab-org/gitlab/-/issues/292748)
1. [Conan](https://gitlab.com/gitlab-org/gitlab/-/issues/293750)
### Default settings
The below table lists the default setting for each package format and whether or not the setting to control duplicates is available.
Each default we set should match the expectation of the de facto public registry, i.e, npmjs.com.
| Package type | Duplicates allowed? (default) | Setting available?|
|--------------|---------------------|---------------------|
| Composer | :question: | :x: |
| conan | :white_check_mark: | :x: |
| Debian | :question: | :x: |
| generic | :white_check_mark: | :white_check_mark: |
| Go | :question: | :x: |
| Helm | :white_check_mark: | :x: |
| maven | :white_check_mark: | :white_check_mark: |
| npm | :x: | :x: |
| nuget | :white_check_mark: | :x: |
| pypi | :x: | :x: |
| RubyGems | :question: | :x: |
| Terraform | :x: | :x: |
### Designs


epic