Fix ListNamespaces responses, centralize NamespaceDescription conversion

[austin-denoble]

Problem

Found another problem with NamespaceDescription responses. IndexConnection.ListNamespaces was not properly attaching Schema or IndexedFields to NamespaceDescription. This was the same issue seen in the CreateNamespace method fixed in a previous PR: #132

Solution

Centralize logic for conversion between db_data_grpc.NamespaceDescription->NamespaceDescription in toNamespaceDescription.
Fix ListNamespaces conversion of NamespaceDescription response objects via toNamespaceDescription. Reuse for create, list, and describe namespace operations.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

CI - unit & integration tests