sec: fix alloc workload identity namespace permission

[dduzgun-security]

Description

Sanitize the Allocations SignedIdentities to prevent privilege escalation within a namespace through unauthorized impersonation of workload associated with ACL policies in any workload within the namespace.
This resolves CVE-2024-12678.

Testing & Reproduction steps

Current tests are passing.

Links

https://github.com/hashicorp/nomad-enterprise/pull/2098

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

[tgross]

LGTM