…ubscription.

http and https SNS topic subscription endpoints require confirmation to set a valid arn otherwise
arn would be set to "pending confirmation". If the endpoints auto confirm then arn is set
asynchronously but if we try to create another subscription with same parameters then api returns
"pending subscription" as arn but does not create another a duplicate subscription. In order to
solve this we should be fetching the subscription list for the topic and identify the subscription
with same parameters i.e., protocol, topic_arn, endpoint and extract the subscription arn.

Following changes were made to support the http/https endpoints that auto confirms

1. Added 3 extra parameters i.e.,
   1. endpoint_auto_confirms -> boolean indicates if end points auto confirms
   2. max_fetch_retries -> number of times to fetch subscription list for the topic to get the subscription arn
   3. fetch_retry_delay -> delay b/w fetch subscription list call as the confirmation is done asynchronously.

  With these parameters help added support http and https protocol based endpoints that auto confirm.

2. Update website doc appropriately

…ion subscription is returned.

"pending confirmation" such as "PendingConfirmation", "Pending Confirmation" etc.

…ields added in the PR

and add the link to amazon describing the auto confirming subscription.

1. Used resource.Retry instead of custom solution
2. Removed unnecessary variables and added required variable to resource.Retry.

…ht value.