Add ScriptSrcs to add extra scripts URLs to load

Summary: This allows importing plugins from local and external hosts.

```
	r.Get("/swagger/*", httpSwagger.Handler(
		httpSwagger.ScriptSrc("/static/swagger-helper.js"),
		httpSwagger.ScriptSrc("https://unpkg.com/[email protected]/build/index.js"),
		httpSwagger.Plugins([]string{"DisableTryItOutWithoutServersPlugin"},
		httpSwagger.URL("http://localhost:1323/swagger/doc.json"), //The url pointing to API definition
	))
```

Test Plan: Updated existing tests. Added example in example/go-chi/

swagger.go

@@ -24,6 +24,7 @@ type Config struct {
 	BeforeScript         template.JS
 	AfterScript          template.JS
 	Plugins              []template.JS
+	ScriptSrcs           []string // URLs to the extra scripts to load
 	UIConfig             map[template.JS]template.JS
 	DeepLinking          bool
 	PersistAuthorization bool
@@ -84,6 +85,13 @@ func Plugins(plugins []string) func(*Config) {
 	}
 }
 
+// Adds `<script src="src"></script>`
+func ScriptSrc(src string) func(*Config) {
+	return func(c *Config) {
+		c.ScriptSrcs = append(c.ScriptSrcs, src)
+	}
+}
+
 // UIConfig specifies additional SwaggerUIBundle config object properties.
 func UIConfig(props map[string]string) func(*Config) {
 	return func(c *Config) {
@@ -259,6 +267,9 @@ const indexTempl = `<!-- HTML for static distribution bundle build -->
 
 <script src="./swagger-ui-bundle.js"> </script>
 <script src="./swagger-ui-standalone-preset.js"> </script>
+{{- range $script := .ScriptSrcs }}
+<script src="{{$script}}"> </script>
+{{- end}}
 <script>
 window.onload = function() {
   {{- if .BeforeScript}}

swagger_test.go

@@ -231,6 +231,13 @@ func TestConfigURL(t *testing.T) {
     // Some plugin
   });`),
 		},
+		{
+			desc: "configure ScriptSrcs",
+			exp: &Config{
+				ScriptSrcs: []string{`/scripts/helper.js`},
+			},
+			cfgfn: ScriptSrc(`/scripts/helper.js`),
+		},
 	}
 
 	for _, fix := range fixtures {
@@ -318,6 +325,8 @@ func TestUIConfigOptions(t *testing.T) {
 
 <script src="./swagger-ui-bundle.js"> </script>
 <script src="./swagger-ui-standalone-preset.js"> </script>
+<script src="./a.js"> </script>
+<script src="./b.js"> </script>
 <script>
 `
 	ftr := `
@@ -336,6 +345,7 @@ func TestUIConfigOptions(t *testing.T) {
 				DocExpansion:         "list",
 				DomID:                "swagger-ui",
 				PersistAuthorization: false,
+				ScriptSrcs:           []string{"./a.js", "./b.js"},
 			},
 			exp: `window.onload = function() {
   
@@ -379,6 +389,7 @@ func TestUIConfigOptions(t *testing.T) {
 					"SomePlugin",
 					"AnotherPlugin",
 				},
+				ScriptSrcs: []string{"./a.js", "./b.js"},
 				UIConfig: map[template.JS]template.JS{
 					"showExtensions":        "true",
 					"onComplete":            `() => { window.ui.setBasePath('v3'); }`,