Use Existing S3 Bucket for "amplify add hosting"

**Is your feature request related to a problem? Please describe.**
We are unable to use existing S3 buckets for hosting the public website created by "amplify add hosting". My organization requires that public website buckets be created in advance by certain teams and following certain procedures/settings; the bucket created by amplify falls outside of this requirement.

**Describe the solution you'd like**
When running "amplify add hosting", would like the prompts to ask whether a new or existing bucket should be created. If an existing bucket is selected, then amplify will not create/modify the bucket. Docs should be updated giving a brief overview of how users should configure the existing bucket in order for it to work with amplify.

**Describe alternatives you've considered**
Alternatives - allow amplify to create a bucket from "amplify add hosting", then delete the newly-created bucket and manually edit the amplify config files to use/point to the pre-existing bucket that we want to use.

**Additional context**
N/A