Replace ClickHouse feature flag check with CH availability check

At present, ClickHouse availablility is tested via feature flags:

• clickhouse_data_collection
• clickhouse_ci_analytics

(perhaps others, I did a quick grep)

Rather than using short-lived feature flags, when testing for the availability of ClickHouse we should have a method on the client:

• The ClickHouse configuration should have at least the main database
• All migrations should have been run (it should have tables and some data)

We could have some method like ClickHouse::Client.available?(:main) which checks for this. Alternatively we could simply check for the existence of the given CH database (main). Other database names can be provided as they are created.

Suggested solution

Add a method with necessary calls that returns whether a particular instance's ClickHouse configuration is ready to be used.

• Suggested: ClickHouse::Client.configured?
• Configuration in https://gitlab.com/gitlab-org/gitlab/-/blob/5774991c8fa1b9970f80f36b95dd2aedc1c7fbb7/config/initializers/click_house.rb#L9 returns an error if it can't be configured so a state can be checked.
• All databases exist (and the migrations are all run)