feat: Support new REST preview service

The REST preview service introduces its own preview token and API
endpoint for Live preview. This values will be used to fetch entry data
for live preview.

Replaced api.contentstack.io to rest-preview.contentstack.com and
replaced  to

fixes: https://contentstack.atlassian.net/browse/VC-311

Author: Deepak-Kharah

config.js

@@ -13,7 +13,7 @@ const config = {
     },
     live_preview: {
         enable: false,
-        host: 'api.contentstack.io'
+        host: 'rest-preview.contentstack.com'
     }
 };
 

index.d.ts

@@ -74,7 +74,8 @@ export interface ContentTypeCollection {
 
 export interface LivePreview {
     host: string
-    management_token: string
+    preview_token?: string
+    management_token?: string
     enable: boolean
 }
 

src/core/lib/utils.js

@@ -242,7 +242,7 @@ export function sendRequest(queryObject, options) {
                 if(queryObject.requestParams.headers['access_token']) 
                     delete queryObject.requestParams.headers['access_token'];
 
-                queryObject.requestParams.headers['authorization'] = queryObject.live_preview.management_token
+                queryObject.requestParams.headers['authorization'] = queryObject.live_preview.preview_token || queryObject.live_preview.management_token;
             } else if(queryObject.live_preview.live_preview) {
                 cachePolicy = 1; // cache then network
             }
@@ -472,7 +472,8 @@ function generateReferenceMap (references) {
 
 async function updateLivePreviewReferenceEntry(referenceMap, entry, stack, options, handlerOptions) {
     const {live_preview:livePreview, requestParams} = stack;
-    const { content_type_uid: livePreviewContentTypeUid, management_token } = livePreview;
+    const { content_type_uid: livePreviewContentTypeUid, preview_token } =
+        livePreview;
 
 
     async function findReferenceAndFetchEntry(referenceMap, entry, setReference) {
@@ -509,7 +510,7 @@ async function updateLivePreviewReferenceEntry(referenceMap, entry, stack, optio
                     stack.requestParams.method = "GET"
 
                     delete stack.requestParams.headers.access_token
-                    stack.requestParams.headers.authorization = management_token
+                    stack.requestParams.headers.authorization = preview_token;
 
                     const data = await Request(stack, options);
                     data.entry._content_type_uid = livePreviewContentTypeUid;