Start using `/v2/appointment/sessions/public/calendarByPin`

[sarthakgupta072]

Using /v2/appointment/sessions/public/calendarByPin is more efficient instead of using /v2/appointment/sessions/public/findByPin.

Make the code changes to support the former endpoint.

After that frequency can be changed to 30 secs also.

Note: Rate limit of cowin API is 100 requests per 5 minutes

(How did I miss this 😭 )