Add several backend filters to the Vue version of the merge request list

What does this MR do and why?

This adds several backend filters for the upcoming Vue merge request list:

* :approver, filter by possible approver of a merge request

• :approved_by, filter by someone who approved a merge request
• :my_reaction_emoji, filter by emoji you've applied
• :merged_by, filter by merger of a merge request
• :release, filter by release tag

The frontend doesn't use these (yet), but for extra credit you could confirm by merging these MRs:

• Add My Reaction filter to Vue merge request lis... (!159807 - merged)
• Add merged-by filter to Vue merge request list app (!159808 - merged)
• Add approved-by filter to Vue merge request lis... (!160005 - merged)

Then enable the vue_merge_request_list feature flag and check the merge requests search bar of a project.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to:

• Add my-reaction filter to Vue merge request lis... (#442920 - closed)
• Add release filter to Vue merge request list app (#442919 - closed)
• Add approved-by filter to Vue merge request lis... (#442916 - closed)
• Add approver filter to Vue merge request list app (#442915 - closed)
• Add merged-by filter to Vue merge request list app (#442914 - closed)

app/graphql/resolvers/merge_requests_resolver.rb

@@ -33,6 +33,30 @@ def self.accept_reviewer
         description: 'Filter by reviewer presence. Incompatible with reviewerUsername.'
     end
 
+    argument :approver, GraphQL::Types::String,
+      required: false,
+      as: :approver_usernames,
+      description: 'Usernames of possible approvers.'
+
+    argument :release, GraphQL::Types::String,
+      required: false,
+      as: :release_tag,
+      description: 'Filter by release tag.'
+
+    argument :approved_by, GraphQL::Types::String,
+      required: false,
+      as: :approved_by_usernames,
+      description: 'Username of the approver.'
+
+    argument :merged_by, GraphQL::Types::String,
+      required: false,
+      as: :merge_user_username,
+      description: 'Username of the merger.'
+
+    argument :my_reaction_emoji, GraphQL::Types::String,
+      required: false,
+      description: 'Filter by your reaction emoji.'
+
     argument :iids, [GraphQL::Types::String],
       required: false,
       description: 'Array of IIDs of merge requests, for example `[1, 2]`.'