feat: implement background syncing for warehouse schema

[shekhar-rudder]

Description

Related to #5347

Refactoring

• Replaced the use of the Schema struct with the SchemaHandler interface, ensuring clients depend on an interface rather than the actual implementation.

New features

• Introduced a new implementation of SchemaHandler, schemaV2, where the schema in the database is the source of truth.
• Added a schema syncer that runs on a configurable schedule (copied from feat: fetch remote schema async #5347).

Implementation Notes

• Retained existing method names to minimize the risk of regression and limit the scope of the PR. These can be updated once the schema implementation is deprecated.
• Controlled the choice between schema and schemaV2 using an env.

Linear Ticket

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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: Patch coverage is 59.00000% with 82 lines in your changes missing coverage. Please review.

Project coverage is 74.73%. Comparing base (8a186e5) to head (f539685).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
warehouse/schema/schema_v2.go	29.06%	60 Missing and 1 partial ⚠️
warehouse/router/sync.go	58.33%	7 Missing and 3 partials ⚠️
warehouse/router/router.go	33.33%	3 Missing and 1 partial ⚠️
warehouse/router/upload.go	69.23%	3 Missing and 1 partial ⚠️
warehouse/schema/schema.go	95.77%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5405      +/-   ##
==========================================
- Coverage   74.82%   74.73%   -0.10%     
==========================================
  Files         438      440       +2     
  Lines       61331    61474     +143     
==========================================
+ Hits        45893    45941      +48     
- Misses      12895    12981      +86     
- Partials     2543     2552       +9     

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