Add wip incremental model

[agnessnowplow]

Opening this PR for an early review to make sure the structure and incremental logic is agreed on before I go ahead and complete / revise it, add the integration tests and make the changes on docs. Currently it is only tested for Snowflake.

Core logic:

• should work out of the box by using the upcoming unified package with the conversions as source and sessions used for path source (both can be changed, needs further testing if we want check examples but due to the field names I think they are pretty much "forced" to use it)
• the paths_to_conversion incremental table collects the latest data in one go (starting from a user defined start date called snowplow_attribution_start_date) taking into account the last conversion event that was processed for incremental runs
• users should be instructed not to change any variables, for safety so far only the incremental_manifest table exists, later we could add warnings to prevent such user errors
• once we have the path data per conversion event we create two other downstream incremental tables the channel and campaign attributions, both of which apply all the different attribution algorithms apart from shapely (need to think about how to support it without python in a future release)
• the report table is currently set as a user configured one with a sample output, this is to allow for custom spend data integration and other flexibility such as including the currently optional paths_to_non_conversion tables which for simplicity's sake is a drop and recompute table for now
• I have left the path summary and channel counts tables for now but I'm not too sure they are needed for anything, we can agree with customers, we might add it as optional models if still useful, not sure

[rlh1994]

Overall structure and approach is good, nice to see how it all flows together. I haven't really had a chance to look at any actual data yet (partly an error with report table), but it all makes sense to go ahead with this approach.

I think we should have a working report table out the box, not including any spend info, but make it easyish to add that in by default. A few other bits here and there but SO much easier to understand and follow than the last one!

[rlh1994]

Feedback :)

[rlh1994]

LGTM, probably!