Allow the user to specify the column name for snowplow_incremental logic

[emielver]

Describe the feature

With the snowplow_incremental logic, users aren't able to specify which column should be used to see which data has been processed. The is_run_with_new_events macro is currently hardcoded to look for start_tstamp, however technically this could be any timestamp column. We should allow users to specify which column they want to point to to see which events have been processed already.

Proposed Solution

In macros/utils/is_run_with_new_events.sql we should replace start_tstamp with a parameter such as {{ start_timestamp }} which should take on a default value of start_tstamp but could be specified by the user in the is_run_with_new_events macro call. We should also create a test to ensure that this is working as expected with custom column names.

[rlh1994]

@emielver We'll cover this in Jan, but is the idea they can choose what is set into start_tstamp and recorded into the manifest?

[emielver]

Closed in #144