Admin enrichment: PCU Groups, CreateDatabase, plus some cleanup/tidiness

[sl-at-ibm]

Fixes #211 .

Additionally,

1. removes the redundant AstraDatabaseCloudProvider, using CloudProviderType consistently everywhere (with case-insensitive deserialization given the variability of the DevOps API responses)
2. moves the following classes to the DataStax.AstraDB.DataApi.Admin namespace:

• RegionInfo
• CreateKeyspaceOptions
• DoesKeyspaceExistOptions
• DropKeyspaceOptions
• ListKeyspacesOptions