Refactor feature flags to use combination of Quarkus config and per-request overrides

[tatu-at-datastax]

So: currently we have just one feature flag for enabling API Tables: it uses generic Quarkus config (via class ApiTablesConfig, setting stargate.tables.enabled). While it works ok, there are some issues:

1. It is not grouped/named as a feature flag (minor -- but we'll get more in future)
2. There is no way to override it on per-request basis

So we would probably want to change things so one could

1. Have default setting (enabled/disabled) for whether API Tables are enabled
2. Allow per-request override via HTTP headers
3. Possibly have "master key" on whether (2) is allowed or not ("enable per-request feature flag overrides)

As to implementation, access to combined feature flag state should be via CommandContext, which can combine necessary information.

[tatu-at-datastax]

Quick note: proposed Config settings:

• Name stargate.feature.flags.XXX (like stargate.feature.flags.tables)
• Value: optional Boolean -- if specified, forces setting; if null/absent, may be overridden on per-request basis

Matching HTTP request header will be:

• Feature-Flag-tables (but being case-insensitive, can send as feature-flag-tables)

[tatu-at-datastax]

Fixed via #1390