✨ sharded-test-server: support for running the caching layer

[p0lyn0mial]

Summary

adds support for running the cache-server to the sharded-test-server binary.
support for caching can be turned on by passing --run-cache-server=true flag, then the test binary will:

1. configure the cache to listen on port 8012 (HTTPS)
2. configure the cache to use embedded-etcd (ports 8010, 8011)
3. configure the cache to use filepath.Join(workingDir, ".kcp-cache") directory
4. run the cache server
5. wait for the cache server's serving certificate
6. use the certificate to create a kubeconfig at the path configured in 3
7. wait until the cache becomes ready

next, the test binary will configure each shard with the following flags:

1. --run-cache-server=true - inherited from the test binary itself
2. --cache-server-kubeconfig-file - that points to the kubeconfig created in step 6. A shard will use the config to establish connection with the cache server

in the future we could run the caching layer by default, for now (until it proves to work), it has to be gated with a flag.

Related issue(s)

part of #342

[sttts]

/lgtm
/approve

[openshift-ci]

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