[Post-99] Additional adjustments to the Admin classes

[sl-at-ibm]

Do not merge before 99. I have put this out only to expedite reviews :)

Fixes #87 .

• Payload to "ListDatabases":
  • fix data types and serialization
  • respect parameters being optional in the payload (i.e. respect fields being omitted)
  • "starting_after" parameter made public (until DB list pagination is managed by the client)
• management of ASTRA_DB_ENVIRONMENT environment variable for tests (needed for some of the items on this PR)
• added a virtual property "Database.Keyspace" (useful for tests but also to client users)
• Astra DB admin: removed in-client keyspace existence check in CreateKeyspace route
• fixed Database's UpdateDBKeyspace implementation (was not working correctly)
  • Extended tests to comprehensively cover the intricacies here (to run manually. I did run them a lot)
• Both DatabaseAdmin* classes' CreateKeyspace:
  • revise overloads: there were boolean/boolean ambiguities with UpdateDBKeyspace and WaitForCompletion Slightly breaking signatures
  • revise defaults: WaitForCompletion had inconsistent defaults across overloads (now always true) Slightly docs-impacting
  • re-order and re-arrange overloads for legibility (and alignment between the two classes)
  • introduce an IDictionary for replicationOptions in the DataAPI class (which affects overloads). Kept overloads minimal. Note: a general dictionary is expected (rationale: it's a very specific use case anyway).
• Fix the hardcoded (prod) DevOps API url from some internal "GetDatabaseAdmin" methods of the AstraDatabasesAdmin class. These are used in the CreateDatabase path.
  • Improved the (manually-run) testing of CreateDatabase to verify. I ran these on PROD as well as DEV to confirm.

@sk-datastax pinging you because of some docs-impacts