Enable Nomad Client to set SI token file for envoy

[shoenig]

As part of #6701 Nomad Clients should be able to configure envoy with an SI token.

gh-6718 covers Nomad Clients being able to request SI tokens be derived on behalf of tasks. Those tokens are written into the secrets directory for each task. Since Nomad provides first class support for using envoy as a Consul Connect sidecar, we can also set this token for envoy when generating the envoy_bootstrap.json file, in the envoy TR hook.

Until Task Dependencies exist, we'll need to think about the existing retry logic here and how it fits in with the token derivation retries.

[github-actions]

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