FE: Add `Policy Type` column to the Policy table

Why are we doing this work

• Currently only one type of policy can be created, Network Policy, but with this epic a user will be able to create a second type of policy, Scan execution.
• Create sortable Policy Type column so a user can see the type of the policies in the table
• Network Policy types should show up as Network in the Policy Type column

Relevant links

Designs:

• https://gitlab.com/groups/gitlab-org/-/uploads/7e0dd6a4bfff1a85672bcf58ecb86d0c/policies-states.png
• #267346[3a.dastpolicies.png]

Non-functional requirements

• Documentation:
• Testing:

Implementation plan

• frontend Create Policy Type column in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/assets/javascripts/threat_monitoring/components/network_policy_list.vue
  • ensure Network Policy types show up as Network in the Policy Type column
  • ensure Scan exectution types show up as Scan exectution in the Policy Type column
  • The column should be sortable.