feat: Implement support for exposing refreshHandler callback to handle token refresh.

[xil222]

Design Doc: link
For this PR, developer has to provide a callback function returns access_token and expire_time both mandatory.
@bojeil-google @bcoe @lsirac Your feedback is highly appreciated!

There're 8 test cases:
getRequestHeader():

1. expired token, calling refreshHandler
2. token not expired, return authorized header with current token
3. no credentials in client, calling refreshHandler

request():

1. one with calling refreshHandler callback case
2. no credentials in client, calling refreshHandler

getAccessToken():

1. call refreshHandler when token expires and no refresh token
2. no credentials in client, calling refreshHandler

RefreshHandler returns no access token:

1. define refresh handler callback with empty access token, rejected when trying to refresh in getAccessToken()

Modify other error message and description in tests due to addition of refreshHandler logic.
Modify other test cases in request() to add another mock for 2nd successful request.

Note:
Since the logic of throwing error is in refreshTokenNoCache I didn’t add "no refreshHandler callback related information" to the error message in the test case

[bcoe]

One nit about the edit in samples/package.json, otherwise this looks reasonable to me.

[bojeil-google]

Hey @xil222 I took a first look. I will continue next week. Thanks.

[xil222]

There're 5 test cases:
getRequestHeader():

1. expired token, calling refreshHandler
2. token not expired, return authorized header with current token

request():

1. one with calling refreshHandler callback case

getAccessToken():

1. call refreshHandler when token expires and no refresh token

RefreshHandler returns no access token:

1. define refresh handler callback with empty access token, rejected when trying to refresh in getAccessToken()

Modify other error message and description in tests due to addition of refreshHandler logic.

Note:
Since the logic of throwing error is in refreshTokenNoCache I didn’t add "no refreshHandler callback related information" to the error message in the test case

[bojeil-google]

The tests look a lot better now. Glad we uncovered this. Thanks for fixing them.