make MAX_SEC_KEY_LEN configurable (as internal setting)

Current limit of MAX_SEC_KEY_LEN is 4K. This should cover majority of use case but it would be good to make this as an internal setting to handle extreme use case.

Please note - when we implement this, it should not be seen as a routine configuration parameter. It is for special cases. In general, the default size limit should be viewed as a design limit for indexes. When documents attributes that need to be indexed exceed this size, we should first consider data model changes or transforms to come within the limit.