api: provide new merge option: `upsert`

[Dentrax]

Is your feature request related to a problem? Please describe.

There should be a new MergeMethod that should put the secret if does not exist, otherwise, it should merge the secret by using patch method like as is.

foo, err := client.KVv2("foo").Patch(context.Background(), "bar", data)
if err != nil {
  // in case an error fallback to the patch way - key does not exist!
  foo, err = client.KVv2("foo").Put(context.Background(), "bar", data)
  if err != nil {
    log.Printf("could not write to Vault: %s: %v\n", variable.Key, err)
     
  }
}

I've tried to pass KVMergeMethodPatch and cas=0 but doesn't solve the problem:

client.KVv2("foo").Patch(context.Background(), "bar", data, vault.WithMergeMethod(vault.KVMergeMethodPatch), vault.WithCheckAndSet(0))

Describe the solution you'd like

Provide a new MergeMethod:

KVMergeMethodUpsert     = "upsert"

Eventually, I would write the following instead:

client.KVv2("foo").Patch(context.Background(), "bar", data, vault.WithMergeMethod(vault.KVMergeMethodUpsert)

Describe alternatives you've considered

-

Explain any additional use-cases -

Additional context -