Backport of Add a sentinel error for missing KV secrets into release/1.11.x

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

Backport

This PR is auto-generated from #16699 to be assessed for backporting due to the inclusion of the label backport/1.11.x.

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

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

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Adding a sentinel error value ErrSecretNotFound. With this change, callers can check specifically for non-existing secrets:

secret, err := client.KvV2("secret").Get("foo")

if errors.Is(err, vault.ErrSecretNotFound) {
    // handle missing secret
}
if err != nil {
    // handle other errors
}

An example where this would be useful is when implementing an "upsert" behavior, as suggested in #16483. The caller can check the error returned from KVv2.Patch for errors.Is(err, vault.ErrSecretNotFound) and call KvV2.Put accordingly.

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Overview of commits

• c8c138f

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