openapi: Make file client more easy to re-use

A few notes about the change:
1. I need to initialize the fileclient once, in an init function, so I
don't have access to `testing.T` yet.
2. I want to be able to configure the openapi files that I use
3. We already have a "cache" client that wraps another client, we don't
need to re-implement caching here, one can just do:
`cache.NewClient(openapitest.NewFileClient("some/path"))` to do a cached
client. Or initialize it in an init/global var.

Since there is still some value to use the embedded file, make an
alternative constructor while using fs.FS interface to be able to
manipulate both virtual and disk-based filesystems.

Kubernetes-commit: 29503fd8d45bc2c9438e92936bf4111162529b40

openapi3/root_test.go

@@ -150,7 +150,7 @@ func TestOpenAPIV3Root_GVSpec(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
-			client := openapitest.NewFileClient(t)
+			client := openapitest.NewEmbeddedFileClient()
 			root := NewRoot(client)
 			gvSpec, err := root.GVSpec(test.gv)
 			if test.err != nil {
@@ -209,8 +209,7 @@ func TestOpenAPIV3Root_GVSpecAsMap(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
-			client := openapitest.NewFileClient(t)
-			root := NewRoot(client)
+			root := NewRoot(openapitest.NewEmbeddedFileClient())
 			gvSpecAsMap, err := root.GVSpecAsMap(test.gv)
 			if test.err != nil {
 				assert.True(t, reflect.DeepEqual(test.err, err))