twilio: add Voice Insights API

Added all three [Twilio Voice Insights API](https://www.twilio.com/docs/voice/insights/api) resources:

- Summary
- Events
- Metrics

How to use it:

```
t.client.Insights.VoiceInsights(callSID).Metrics.GetPage(ctx, params)

t.client.Insights.VoiceInsights(callSID).Events.GetPage(ctx, params)

t.client.Insights.VoiceInsights(callSID).Summary.Get(ctx)
t.client.Insights.VoiceInsights(callSID).Summary.GetPartial(ctx)
```

AUTHORS.txt

@@ -3,6 +3,7 @@ Andrey Mak <andrey.mak@yahoo.com>
 Duncan Crawford <duncan@noughtsandones.co.uk>
 Elijah Oyekunle <eloyekunle@gmail.com>
 Fabrizio Moscon <mosconfabrizio@gmail.com>
+Joey Lee <jq@yeoji.com>
 Josh Hornby <hello@joshhornby.co.uk>
 Kelmer Perez <kperez@wyzant.com>
 Kevin Burke <kevin@burke.dev>

README.md

@@ -99,6 +99,7 @@ The API is unlikely to change, and currently covers these resources:
   - Workflows
 - Transcriptions
 - Wireless
+- Voice Insights
 - Access Tokens for IPMessaging, Video and Programmable Voice SDK
 
 ### Error Parsing

http.go

@@ -67,6 +67,11 @@ var TaskRouterBaseUrl = "https://taskrouter.twilio.com"
 
 const TaskRouterVersion = "v1"
 
+// Voice Insights service
+var InsightsBaseUrl = "https://insights.twilio.com"
+
+const InsightsVersion = "v1"
+
 type Client struct {
 	*rest.Client
 	Monitor    *Client
@@ -78,6 +83,7 @@ type Client struct {
 	Verify     *Client
 	Video      *Client
 	TaskRouter *Client
+	Insights   *Client
 
 	// FullPath takes a path part (e.g. "Messages") and
 	// returns the full API path, including the version (e.g.
@@ -134,6 +140,9 @@ type Client struct {
 
 	// NewTaskRouterClient initializes these services
 	Workspace func(sid string) *WorkspaceService
+
+	// NewInsightsClient initializes these services
+	VoiceInsights func(sid string) *VoiceInsightsService
 }
 
 const defaultTimeout = 30*time.Second + 500*time.Millisecond
@@ -269,6 +278,28 @@ func NewTaskRouterClient(accountSid string, authToken string, httpClient *http.C
 	return c
 }
 
+func NewInsightsClient(accountSid string, authToken string, httpClient *http.Client) *Client {
+	c := newNewClient(accountSid, authToken, InsightsBaseUrl, httpClient)
+	c.APIVersion = InsightsVersion
+	c.VoiceInsights = func(callSid string) *VoiceInsightsService {
+		return &VoiceInsightsService{
+			Summary: &CallSummaryService{
+				callSid: callSid,
+				client:  c,
+			},
+			Metrics: &CallMetricsService{
+				callSid: callSid,
+				client:  c,
+			},
+			Events: &CallEventsService{
+				callSid: callSid,
+				client:  c,
+			},
+		}
+	}
+	return c
+}
+
 // NewPricingClient returns a new Client to use the pricing API
 func NewPricingClient(accountSid string, authToken string, httpClient *http.Client) *Client {
 	c := newNewClient(accountSid, authToken, PricingBaseURL, httpClient)
@@ -345,6 +376,7 @@ func NewClient(accountSid string, authToken string, httpClient *http.Client) *Cl
 	c.Verify = NewVerifyClient(accountSid, authToken, httpClient)
 	c.Video = NewVideoClient(accountSid, authToken, httpClient)
 	c.TaskRouter = NewTaskRouterClient(accountSid, authToken, httpClient)
+	c.Insights = NewInsightsClient(accountSid, authToken, httpClient)
 
 	c.Accounts = &AccountService{client: c}
 	c.Applications = &ApplicationService{client: c}
@@ -434,6 +466,9 @@ func (c *Client) UseSecretKey(key string) {
 	if c.Wireless != nil {
 		c.Wireless.UseSecretKey(key)
 	}
+	if c.Insights != nil {
+		c.Insights.UseSecretKey(key)
+	}
 }
 
 // GetResource retrieves an instance resource with the given path part (e.g.