[HOTFIX][BE]: Fix unmitigated Guzzle timeout error (#294)

Changelog:
- Add timeout into http request (5 minutes) by default, this to mitigate
default http timeout from guzzle (30 seconds). Due un-predicted Aspose
API response
- Add handler when timeout already running out to return proper error
message and avoid backend server crashes
- Merge function convert to xlsx and pptx into one function