feat(kuma-cp): enrich policies endpoint

[jakubdyszkiewicz]

• rename displayName to pluralDisplayName
• add singularDisplayName
• remove ExternalService from the list of policies
• add isExperimental

Checklist prior to review

• Link to docs PR or issue -- 341
• Link to UI issue or PR --
• Is the issue worked on linked? --
• Unit Tests --
• E2E Tests --
• Manual Universal Tests --
• Manual Kubernetes Tests --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? --

[codecov-commenter]

Codecov Report

Merging #4791 (9fccef6) into master (4c18e48) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4791      +/-   ##
==========================================
+ Coverage   46.44%   46.45%   +0.01%     
==========================================
  Files         690      690              
  Lines       47086    47088       +2     
==========================================
+ Hits        21870    21876       +6     
+ Misses      23290    23288       -2     
+ Partials     1926     1924       -2     

Impacted Files	Coverage Δ
api/mesh/v1alpha1/gateway.pb.go	6.85% <ø> (ø)
api/mesh/v1alpha1/gateway_route.pb.go	3.55% <ø> (ø)
api/mesh/v1alpha1/retry.pb.go	7.44% <ø> (ø)
...core/resources/apis/mesh/zz_generated.resources.go	16.06% <ø> (ø)
pkg/api-server/policies_endpoints.go	89.47% <100.00%> (+1.23%)	⬆️
pkg/defaults/components.go	53.33% <0.00%> (-3.34%)	⬇️
pkg/xds/cache/once/once.go	100.00% <0.00%> (+5.00%)	⬆️
pkg/xds/cache/once/cache.go	94.87% <0.00%> (+12.82%)	⬆️

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