MB-61890 - Introducing config at zap layer

[Thejas-bhat]

• The PR introduces the concept of config which can be index specific config values at the zap level. This would help in the future to perform any index specific tuning at the zap level.
• This involves extending the existing interface's Open and New APIs.
• The config is a member field in segmentBase struct but not persisted as part of segment format.