Allow OR filtering for labels on epics list

What does this MR do and why?

This MR adds OR filtering for label on epic lis

• When the or_issuable_queries feature flag is enabled:
  • User can perform OR filtering for labels
  • The operators are shown with friendly text (e.g. is)
• When the or_issuable_queries feature flag is disabled:
  • User cannot perform OR filtering for labels
  • The operators are shown as symbols (e.g. =)

No changelog required as it is behind the feature flag or_issuable_queries, defaulted off

Screenshots or screen recordings

Epics___Gitlab_Org___GitLab__2_

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:or_issuable_queries)

2. Visit any group epics list pages such as http://127.0.0.1:3000/groups/gitlab-org/-/epics

3. Test labels OR filtering in the filtered search bar

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #382969 (closed)