WIP: 🐛 authentication: add custom SA lookup with ttl cache for non-local clusters

[sttts]

Summary

With service accounts valid (even as system:anonymous+system:authenticated) on other clusters, we have to validate them on other shards. As there are many service accounts, we cannot distribute them via the cache server. But luckily there are not many those service accounts. We can use some ttl cache for that and use a real external client.

TODO

• wire external client

Related issue(s)

Fixes # #3289

Release Notes

NONE

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@sttts: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kcp-test-e2e	82d13e5	link	true	/test pull-kcp-test-e2e
pull-kcp-test-e2e-sharded	82d13e5	link	true	/test pull-kcp-test-e2e-sharded
pull-kcp-test-e2e-shared	82d13e5	link	true	/test pull-kcp-test-e2e-shared
pull-kcp-test-e2e-multiple-runs	82d13e5	link	true	/test pull-kcp-test-e2e-multiple-runs
pull-kcp-test-unit	82d13e5	link	true	/test pull-kcp-test-unit
pull-kcp-lint	82d13e5	link	true	/test pull-kcp-lint

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[kcp-ci-bot]

PR needs rebase.

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