allow access to cache object through context

this surfaces the struct kr_cache through context variable,
it doesn't implement any API or documented interface for it,
so I just added a tests for the struct presence and introspection
11 jobs for vendemiat/extend_kres_lua in 6 minutes and 55 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed #94595
docker linux amd64
build:linux:amd64

00:00:24

 
  Test
passed #94599
docker linux amd64
deckard:linux:amd64

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passed #94598
docker linux amd64
installcheck:linux:amd64

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passed #94600
docker linux amd64
installcheck:valgrind:linux:amd64

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passed #94596
docker
lint:lua

00:00:14

passed #94603
docker linux amd64
respdiff:iter:tcp:linux:amd64

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passed #94604
docker linux amd64
respdiff:iter:tls:linux:amd64

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passed #94602
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respdiff:iter:udp:linux:amd64

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passed #94597
docker linux amd64
test:linux:amd64

00:00:24

passed #94601
docker linux amd64
test:linux:amd64:valgrind

00:01:11

 
  Coverage
passed #94605
docker linux amd64
coverage

00:00:20

68.7%