[Vault-5736] Add (*Client).WithNamespace() for temporary namespace handling

[VinnyHC]

Fixes: #14934

In the API Client should have a method to quickly set (or unset) namespaces on calls. Helpful for calls that are intended to be issued against the root.

Current flow for switching namespaces:

	// Create a Client
	nsClient, err := NewClient(DefaultConfig())
	nsClient.SetNamespace("sample-namespace")

	// Clone the client
	rootClient, err := nsClient.Clone()
	// Clear the namespace
	rootClient.ClearNamespace()
	// Make the call
	rootClient.Sys().Health()

Improved flow:

	// Create a client
	nsClient, err := NewClient(DefaultConfig())
	nsClient.SetNamespace("my-ns")
	
	// issued without a namespace
	nsClient.WithNamespace("").Sys().Health()
	
	// issued with X-Vault-Namespace: other-ns
	nsClient.WithNamespace("other-ns").Logical.Read()
	
	// nsClient retains original namespace
	nsClient.Logical.Read()