Backport of Ensure a consistent TLS configuration into release/vault-1.11.x

[hc-github-team-secure-vault-ecosystem]

Backport

This PR is auto-generated from #173 to be assessed for backporting due to the inclusion of the label backport/vault-1.11.x.

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merge conflict error: POST https://api.github.com/repos/hashicorp/vault-plugin-auth-kubernetes/merges: 409 Merge conflict []

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

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Previously, it was possible for the http.Client's Transport to be missing the necessary root CAs to ensure that all TLS connections between the auth engine and the Kubernetes API were validated against a configured set of CA certificates.

This fix ensures that the http.Client's Transport is always consistent with the configured CA cert chain, by introducing a periodic TLS configuration checker that is started as part of the backend's initialization. The periodic checker provides a signal handler that will update the backend's TLS configuration on SIGHUP.

Other fixes:

• only update the client's transport when the CA certificate pool has changed.

Closes #169 #170

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Overview of commits

• 6fb49ec

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