[CBM] Custom endpoints without a scheme now return an error

Problem

Raised in https://www.couchbase.com/forums/t/issues-with-backups-on-couchbase-version-7-6-2

Using a custom AWS endpoint without specifying HTTP/HTTPS e.g.

cbbackupmgr config --obj-endpoint s3.amazonaws.com

worked prior to 7.6.2. Testing suggests that the AWS SDK was automatically prepending "https://" to the endpoint (need to confirm this). 

In MB-61223 (7.6.2), the AWS SDK used by cbbackupgmr was upgraded. The new SDK seems to instead return the following error when given the above invalid URI instead:

operation error S3: PutObject, resolve auth scheme: resolve endpoint: endpoint rule error, Custom endpoint `<endpoint-without-scheme>` was not a valid URI

Fix

Modify the URL before passing it to the SDK such that URI's without schemes give the same result as they did before 7.6.2.