🌱 Add experimental package and shared cache & informer implementation

[mjudeikis]

This PR goes a bit wild. This is an experimental package (/exp) and an example that implements a shared cache & informer for use with multicluster-runtime.

The benefit is that we enable memory and cross-cluster queries for the reconciler (see exp-kind-shared-cache).

In a way, this is currently not possible to be built using upstream client-go packages due to hardcoded KeyFunc usage (it's strange why we have those but we can't use them as they are not exposed :) )

90% of the code here is copied from the client-go tooling for informers and cache.

This is, in a way, idea seeding effort to see if people would find this useful

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mjudeikis

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [mjudeikis]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[mjudeikis]

@sttts gentle reminder :)

[embik]

I'm not sure I like this being in an exp package, even though I understand the intention. If we ever "graduate" this, we will effectively break updates for projects already importing it. It will look like there is no newer version available, and devs will manually figure out it moved places.

I would be more in favor of putting sufficient module-level and maybe even function/struct-level doc strings on this marking it as an experimental package under a permanent package path.