Enable Nomad Client to request and inject Service Identity tokens

[shoenig]

As part of #6701 Nomad Clients will need to be able to

• use a task pre-start hook to identify connect-enabled services
• request necessary SI tokens from the Nomad Servers
• inject the SI tokens into tasks
  • write to task secrets directory
  • pass token into envoy using -token-file argument (split into Enable Nomad Client to set SI token file for envoy #6789)

[github-actions]

I'm going to lock this issue because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.