feat: use append vs merge option from backend config

[fracasula]

Description

Adding support for opt-in deduplication.

Linear Ticket

< Linear Link >

E2E Warehouse run

https://github.com/rudderlabs/e2e/actions/runs/6587296799

Updated Flow Diagram

Outstanding questions

• do we want to have a shouldMerge method in mssql as well here like we do in postgres? if we add it, is it OK for existing customers that are using mssql?
• should we review also other destinations like azure synapse and add a shouldMerge there as well?

TODOs

• document CanAppend with a generic statement in the public docs saying that even though the user didn't opt-in with enableMerge we might merge anyway due to other conditions e.g. if they connected an extract source, we replay the data, etc...
• check if we have live configurations for our customers that are using loadTableStrategy and replace that with enableMerge
  • done here: https://github.com/rudderlabs/rudder-devops/pull/9020

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (72423dd) 71.64% compared to head (e738f57) 71.26%.
Report is 1 commits behind head on master.

❗ Current head e738f57 differs from pull request most recent head a1b912e. Consider uploading reports for the commit a1b912e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3965      +/-   ##
==========================================
- Coverage   71.64%   71.26%   -0.38%     
==========================================
  Files         372      370       -2     
  Lines       54977    54083     -894     
==========================================
- Hits        39389    38544     -845     
+ Misses      13262    13231      -31     
+ Partials     2326     2308      -18     

Files	Coverage Δ
testhelper/clone.go	100.00% <100.00%> (ø)
warehouse/integrations/mssql/mssql.go	76.96% <100.00%> (-0.07%)	⬇️
warehouse/integrations/postgres/load.go	87.27% <100.00%> (-2.85%)	⬇️
warehouse/integrations/postgres/postgres.go	75.98% <100.00%> (+0.07%)	⬆️
warehouse/internal/model/warehouse.go	100.00% <100.00%> (ø)
warehouse/router/router.go	87.52% <100.00%> (-0.05%)	⬇️
warehouse/utils/querytype.go	100.00% <100.00%> (ø)
warehouse/integrations/deltalake/deltalake.go	78.43% <88.88%> (-0.06%)	⬇️
warehouse/integrations/snowflake/snowflake.go	66.51% <88.88%> (-1.66%)	⬇️
warehouse/integrations/bigquery/bigquery.go	63.62% <72.00%> (-0.75%)	⬇️
... and 1 more

... and 53 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fracasula]

@lvrach @achettyiitr hey guys, please review also the flow diagram and the outstanding questions (see PR description).