allow user to pass content topic to createSubscription

[adklempner]

This is a feature or change request

Problem

Currently, creating a subscription via filter requires passing an instance of SingleShardInfo. This is not user friendly when using autosharding. The user should be able to just pass in the content topic in order to create a subscription.

Proposed Solutions

Modify createSubscription to accept a content topic and convert it to the proper pubsubTopic within the function itself.

Notes

Related to #1500

[chair28980]

Related to #1814

[danisharora099]

creating a subscription via filter requires passing an instance of SingleShardInfo

it takes either SingleShardInfo or the type PubsubTopic which is string, which seems to be relevant for autosharding. what am i missing?