Add GET /api/v1/accounts/lookup REST API (#15740)

app/controllers/api/v1/accounts/lookup_controller.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class Api::V1::Accounts::LookupController < Api::BaseController
+  before_action -> { authorize_if_got_token! :read, :'read:accounts' }
+  before_action :set_account
+
+  def show
+    render json: @account, serializer: REST::AccountSerializer
+  end
+
+  private
+
+  def set_account
+    @account = ResolveAccountService.new.call(params[:acct], skip_webfinger: true) || raise(ActiveRecord::RecordNotFound)
+  end
+end