Refactor counter implementations to be polymorphic

What does this MR do and why?

This is a refactor MR to make increments on project statistics easier to follow. There is no change to any apparent behaviour. This MR sets up the groundwork for the following MR to fix the counter refresh (still in draft). !104216 (diffs)

1. #increment_counter is now defined within CounterAttribute. It will toggle between the legacy ActiveRecord counter and the Redis backed counter, now called BufferedCounter, based on the defined list of counter_attributes.
2. Add 2 new classes to encapsulate increment operations:
   1. Gitlab::Counters::BufferedCounter is the Redis backed counter, with most of the Redis implementation contained in this class. There are still some left in CounterAttribute. This change will follow.
   2. Gitlab::Counters::LegacyCounter is a wrapper around the legacy ActiveRecord counter. It simply delegates back to the ActiveRecord.

As a result, from the perspective of a client of ProjectStatistics such as various services, there is no need to distinguish how a counter for a particular attribute is implemented, be it Redis or ActiveRecord.

This is also needed so that more complex Redis implementation needed to fix https://gitlab.com/gitlab-org/gitlab/-/issues/373469 can be contained within the Redis counter and be more easily tested.

Follow ups to be done in following MRs

• clean up remaining counter_attribute_enabled? in various methods. This enabled check should only be needed when deciding which counter to use.
• avoid exposing Redis keys to flush_increments_to_database! if possible.

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Related to #373469