Backport of Update vault ca provider namespace configuration into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #19095 to be assessed for backporting due to the inclusion of the label backport/1.15.

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The person who merged in the original PR is:
@cthain
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

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

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Description

This PR fixes the regression described in #19051. The Vault CA provider has been updated to only set the namespace on the Vault client if it is not empty, otherwise it falls back to the base namespace configured for the provider.

Testing & Reproduction steps

A new unit test TestVaultCAProvider_EnterpriseNamespace has been created to exercise the update. This requires a local Vault Enterprise binary and license to run, otherwise the test will automatically be skipped.

$ vault version
Vault v1.15.0+ent (d3729711f875a9dedea802079cd7f0e4b1d6e8d5), built 2023-09-22T21:04:53Z
$ export VAULT_LICENSE=$(cat /path/to/vault.license)
$ go test ./agent/connect/ca -run '^TestVaultCAProvider_EnterpriseNamespace$'
ok  	github.com/hashicorp/consul/agent/connect/ca	1.916s

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• 6d3a7fd - 8868f67

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[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR

[cthain]

Closing in favour of manual backport #19147