Backport of Verify TLS certificate on endpoints that are used between agents only into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #11956 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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There are different types of endpoints in Nomad, and different types use different mechanism to validate and authenticate requests.

The most common endpoints are called by end-users via the HTTP API and get translated to an RPC request. They are authenticated using ACL tokens.

Another type of endpoint is used by Raft and are called between servers. This process doesn't have ACL tokens involved, so requests are validated by checking the mTLS certificates, when available.

Finally, there are endpoints that are called by agents directly via RPC, either client -> server or server -> server. These also don't use ACL tokens, so there was no explicit validation.

This PRs adds a check to these agent to agent endpoints similar to Raft's, using the mTLS certificate, if available.

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[lgfa29]

Manually backported

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