Set Archival URI in resource state from cluster namespace configuration

[caffalaughrey]

Description

Fixes #54.

• Setting URI values in state following create and update
• Removing empty string as default value for URI attributes

TODOs

• Generate the docs.
• Run the relevant tests successfully.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

• Added history_archival_uri and visibility_archival_uri to state from namespace describe config following create/update
• Removed default value of stringdefault.StaticString("") for these attributes in favour of implicit null

Migration Guide

In cases where state drift is already happening due to the original issue, terraform apply resulting in change action for the resource will resolve.