refactor: replace schema struct with interface to enable flexible implementation

[shekhar-rudder]

Description

Refactoring

• Replaced the Schema struct with the SchemaHandler interface, ensuring that clients depend on the interface rather than the concrete implementation. This change will enable switching to a different schema implementation via a feature flag.
• Schema -> schema

Linear Ticket

resolves WAR-150

Security

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

[achettyiitr]

Can we use a manager in here, instead of directly accessing redshift package?

Suggested change

	rs := redshift.New(config.New(), logger.NOP, stats.NOP)
	whManager, err := manager.New(warehouseutils.POSTGRES, conf, logger.NOP, statsStore)
	require.NoError(t, err)

[shekhar-rudder]

Replaced redshift with manager. I had used redshift because that is how other tests were written

[achettyiitr]

Yeah, we would need to fix other tests as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.78%. Comparing base (b282c33) to head (e4a1e9e).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5426      +/-   ##
==========================================
- Coverage   74.81%   74.78%   -0.03%     
==========================================
  Files         440      440              
  Lines       61639    61668      +29     
==========================================
+ Hits        46115    46120       +5     
- Misses      12987    13006      +19     
- Partials     2537     2542       +5     

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