Backport of vault: detect namespace change in config reload into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #14298 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

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

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The namespace field was not included in the equality check between old and new
Vault configurations, which meant that a Vault config change that only changed
the namespace would not be detected as a change and the clients would not be
reloaded.

Also, the comparison for boolean fields such as enabled and
allow_unauthenticated was on the pointer and not the value of that pointer,
which results in spurious reloads in real config reload that is easily missed in
typical test scenarios.

Includes a minor refactor of the order of fields for Copy and Merge to match
the struct fields in hopes it makes it harder to make this mistake in the
future, as well as additional test coverage.

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This comes out of #14233 but doesn't fix it as far as I can tell.

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