🌱 Cache DiscoveryVariables calls

[fabriziopandini]

What this PR does / why we need it:
DiscoveryVariables is expected to return a "static" response + usually there are few extensions in a cluster, however today we keep calling DiscoveryVariables at every CC reconcile, and this could become very noisy if you have many CC.

This PR introduces a cache that prevents those repeated calls, all getting the same answer, to happen.

/area runtime-sdk

[fabriziopandini]

/test

[k8s-ci-robot]

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/test

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[fabriziopandini]

/test pull-cluster-api-e2e-main

[sbueringer]

/test pull-cluster-api-e2e-main

[sbueringer]

@chrischdi @fabriziopandini Should be ready for another round of review

/hold
for additional unit tests in client.go

[sbueringer]

/cherry-pick release-1.8

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.8 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.8

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[sbueringer]

/test pull-cluster-api-e2e-main
/hold cancel

[sbueringer]

/cherry-pick release-1.9

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.9 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.9

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[chrischdi]

/retest

KCP adoption flake

[fabriziopandini]

@sbueringer thanks for the commit on this PR, looks great!

[chrischdi]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 33847b7c6ccd6b35d06d140c760db41a07b97ee1

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrischdi

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[sbueringer]

I don't see how this failure could be related: https://prow.k8s.io/view/gs/kubernetes-ci-logs/pr-logs/pull/kubernetes-sigs_cluster-api/11592/pull-cluster-api-e2e-main/1869761529562271744

How stable are the e2e tests at the moment?

/retest

[k8s-infra-cherrypick-robot]

@sbueringer: #11592 failed to apply on top of branch "release-1.8":

Applying: Cache DiscoveryVariables calls
Using index info to reconstruct a base tree...
M	internal/controllers/clusterclass/clusterclass_controller.go
M	internal/controllers/clusterclass/clusterclass_controller_test.go
M	internal/controllers/topology/cluster/patches/external/external_patch_generator_test.go
M	test/extension/handlers/topologymutation/handler_integration_test.go
Falling back to patching base and 3-way merge...
Auto-merging test/extension/handlers/topologymutation/handler_integration_test.go
CONFLICT (content): Merge conflict in test/extension/handlers/topologymutation/handler_integration_test.go
Auto-merging internal/controllers/topology/cluster/patches/external/external_patch_generator_test.go
Auto-merging internal/controllers/clusterclass/clusterclass_controller_test.go
CONFLICT (content): Merge conflict in internal/controllers/clusterclass/clusterclass_controller_test.go
Auto-merging internal/controllers/clusterclass/clusterclass_controller.go
CONFLICT (content): Merge conflict in internal/controllers/clusterclass/clusterclass_controller.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"
Patch failed at 0001 Cache DiscoveryVariables calls

In response to this:

/cherry-pick release-1.8

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[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #11599

In response to this:

/cherry-pick release-1.9

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