feat: add support for managed identities (#61)

BREAKING CHANGE: replaces `TaskClient.create` with `TaskClient.createFromKey` and `TaskClient.createFromCredential`

README.md

@@ -55,13 +55,23 @@ const util = require('util');
 
 async function main() {
   // Set up the client
-  const client = await TaskClient.create(
+  const client = await TaskClient.createFromKey(
     'https://your-cosmos-account.documents.azure.com:443/',
     'your-cosmos-database',
     'your-cosmos-collection', // Will be created if it doesn't exist
     'base64-encoded key'
   );
 
+  /*
+   *  // Alternatively initialize the client with a managed identity.
+   *  const client = await TaskClient.createFromCredential(
+   *    'https://your-cosmos-account.documents.azure.com:443/',
+   *    'your-cosmos-database',
+   *    'your-cosmos-collection', // Will be created if it doesn't exist
+   *    new ChainedTokenCredential(...) // From @azure/identity library.
+   *  );
+   */
+
   // Create a task that runs once
   const task = await client.create('sample-task', { hello: 'world' });
 
@@ -104,7 +114,7 @@ Let's break down what's going on in here:
 Before we get going, we need a client to work with.
 
 ```js
-const client = await TaskClient.create(
+const client = await TaskClient.createFromKey(
   'https://your-cosmos-account.documents.azure.com:443/',
   'your-cosmos-database', // Will be created if it doesn't exist
   'your-cosmos-collection', // Will be created if it doesn't exist