Filter by custom field select values

What does this MR do and why?

1. Add filtering for custom fields on issue list
2. Add EE overrides for convertToUrlParams, convertToApiParams, and some of their friends

Screenshots or screen recordings

How to set up and validate locally

Easy way:

Apply diff

diff --git a/ee/spec/features/issues/filtered_search/filter_issues_by_custom_fields_spec.rb 
b/ee/spec/features/issues/filtered_search/filter_issues_by_custom_fields_spec.rb
index 3b67d35e473a..d41f42a0701a 100644
--- a/ee/spec/features/issues/filtered_search/filter_issues_by_custom_fields_spec.rb
+++ b/ee/spec/features/issues/filtered_search/filter_issues_by_custom_fields_spec.rb
@@ -31,6 +31,8 @@
 
         click_filtered_search_bar
 
+        live_debug
+
         aggregate_failures do
           within_testid('filtered-search-input') do
             expect(page).to have_content(select_field.name)

WEBDRIVER_HEADLESS=0 bin/spring rspec ee/spec/features/issues/filtered_search/filter_issues_by_custom_fields_spec.rb:29

Other way

1. Enable custom_fields_feature flag
2. Add a Single or Multi select custom field for a top level group
3. Go to Issue list list page
4. Filter by the custom field

MR acceptance checklist

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