Add a sentinel error for missing KV secrets

[averche]

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.

[digivava]

Makes sense to me, and good improvements to some tests as well. Thank you!