[CT-3501] [Epic] Streamline Incremental Strategies

[dbeatty10]

This epic comprises the changes to Core + Adapters necessary to streamline incremental strategies as described in #5245:

1. Enable advanced end users to supply their own custom incremental strategies.
2. Enable adapter maintainers to more efficiently support their chosen subset of the built-in incremental strategies (+ custom strategies):
   • append
   • delete+insert
   • merge
   • insert_overwrite

Core

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1. [CT-646] [Proposal] Streamline Incremental Strategies #5245
   enhancement incremental +2
   

dbt-postgres

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1. [CT-646] [Proposal] Streamline Incremental Strategies #5245
   enhancement incremental +2
   

dbt-redshift

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1. Enable Redshift adapter to work with incremental materialization refactor (inherit incremental tests, etc.)

dbt-snowflake

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1. [CT-840] Enable Snowflake adapter to work with incremental materialization refactor dbt-snowflake#195
   enhancement +1

dbt-bigquery

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1. [CT-889] Minimal changes to support dbt Core incremental materialization refactor dbt-bigquery#232
   enhancement +1
2. [CT-958] Refactor incremental materialization, using new incremental_strategy foundations dbt-bigquery#242
   tech_debt +1
3. [CT-2073] Full compatibility with incremental materialization refactor in v1.3 dbt-bigquery#523
   enhancement +1
4. [Feature] delete partitions, fast & free (?) dbt-bigquery#1166
   enhancement partitioning triage +3

dbt-spark

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1. [CT-888] Minimally support incremental materialization refactor dbt-spark#402
   enhancement +1
2. [CT-957] Refactor incremental materialization, using new incremental_strategy foundations dbt-spark#411
   tech_debt +1

Documentation

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1. New components for incremental strategies docs.getdbt.com#1761
   adapters content improvement incremental models priority: high size: large +6
   
2. User-defined custom incremental strategies docs.getdbt.com#4716
   January-2024 content size: medium +3