External ServiceAccount token signer

<div class="stage" data-phase="0"><div class="title">TOKEN SIGNING BOUNDARY</div><div class="signflow"><div class="panel api">API SERVER<small>token request</small></div><div class="wire"><span>→</span><span>←</span></div><div class="panel signer">EXTERNAL SIGNER<small>private key</small></div></div><div class="panel verify">PUBLIC KEY · verify pending</div></div>

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