Feature: support for arbitrary predicates for incremental models

[dave-connors-3]

resolves #3293

Description

This PR adds an optional configuration to incremental models that allows the user to supply an arbitrary filter expression to merge/delete statements to reduce the amount of data scanned while performing those operations.

V1 of this logic allows the user to pass a list of dictionaries to the new incremental predicates config to supply a column and expression to evaluate at runtime. The dictionary format allows proper table aliasing in these statements.

{{
    config(
        materialized='incremental',
        unique_key = 'customerorderid',
        incremental_predicates = [{
            'source_col' : 'customerorderid',
            'expression' : '> 1'
        }]
        
    )
}}

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change

[oolongtea]

is there a reason to call the parameter source_col?

I'd expect something like target_col or target_column to clarify that the predicate is applied to the target table in the context of an incremental load into a target model (sometimes src or source is used as alias in a merge statement)

[oolongtea]

does this mean incremental predicates will only work for delete+insert strategy? is this just to get the feature started with least amount of work, or is there a blocker to implement for the merge strategy?

I don't exactly have an exact use case to share, so just curious and generally thinking about pros/cons of moving from merge to delete+insert strategy.

[dave-connors-3]

@jtcohen6 -- I think it's fair to say this is a bit outdated -- i had a new branch going most recently that I can use to consolidate with @NiallRees's open PR, so we can close this one!