✨ POC for mounts (Master PR, not for merging)

[mjudeikis]

Summary

This PR is a working example of workspace mounts mounting remote cluster on top of it.

• In general everything in contrib directory in the long term will go to the separate repository (We can keep it here if people agree too).
• Any code in core would need to be split and merged as smaller prs.

In addition this enables core to run with bit more flexible mini-front-proxy, as I found very hard to develop external VirtualWorskapces using kcp start so kinda took it and extended.

This add example of following:

mjudeikis@unknown2:/go/src/github.com/kcp-dev/kcp/contrib/mounts-vw$ k ws tree
.
└── root
    ├── consumer
    │   └── kind-cluster
    ├── operators
    │   └── mounts
    └── providers
        └── mounts

where:
root:providers:mounts is where all APIExports lives and where code operators are looking at.
root:operators:mounts is where people operators created their bindings and created TARGET cluster objects (separate binding)
root:consumers where consumer bounded MOUNT apiexport and created a single mount ws kind-cluster with mount backed by Target API.

In addition when bootstrapping shard-aware operators - it's painful and hard to make miskates. I did some skeleton but I think we need a framework for this.

In general this this WIP PR, to be split to many many smalls ones.

#3188
#3183

Related issue(s)

#3189 as follow-up

Release Notes

NONE

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