FE: Add `Type` filter for Policy table

Why are we doing this work

• The policy table will have many different types of policies
• Users will want to be able to filter by policy type in the policy table

Relevant links

Design: #267346[3a.dastpolicies.png]

Non-functional requirements

• [-] Documentation
  • [-] Document how to filter by policy type?
• Testing
  • Filter component renders supported options.
  • Policy list filters items properly.
  • Does not trigger unnecessary GraphQL queries.

Implementation plan

• frontend Allow for users to filter policies by the Type column in the Policy table in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/assets/javascripts/threat_monitoring/components/network_policy_list.vue
  • Move environment_picker.vue to a filters/ subdirectory.
  • Create a PolicyTypeFilter component. Consider leveraging ee/app/assets/javascripts/security_configuration/components/dropdown_input.vue.
    • It relies on a list of supported policy types (defined as constants client-side) to render options.
  • The policy list syncs a local state property via a v-model.
  • Policies are filtered client-side based on the filter's value.
  • GraphQL queries should be skipped when the filter's value would prevent their result from being displayed.