refactor: sources async job

[achettyiitr]

Description

• Introduce the repo and model for the Source Job.
• Start() to honour context.
• Minor cleanup to use SourceJob keyword instead of AsyncJob.
• Introduce SourceManager for the source package.

Linear Ticket

• Resolved PIPE-168

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: 141 lines in your changes are missing coverage. Please review.

Comparison is base (17590a6) 71.53% compared to head (98a2ca2) 71.73%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4008      +/-   ##
==========================================
+ Coverage   71.53%   71.73%   +0.19%     
==========================================
  Files         373      373              
  Lines       54838    54941     +103     
==========================================
+ Hits        39227    39410     +183     
+ Misses      13273    13199      -74     
+ Partials     2338     2332       -6     

Files	Coverage Δ
warehouse/api/http.go	78.94% <100.00%> (ø)
warehouse/app.go	89.59% <100.00%> (ø)
warehouse/integrations/testhelper/setup.go	99.36% <100.00%> (-0.32%)	⬇️
warehouse/internal/model/upload.go	100.00% <ø> (ø)
warehouse/internal/repo/upload.go	91.01% <ø> (ø)
warehouse/router/router.go	87.54% <100.00%> (-0.03%)	⬇️
warehouse/integrations/testhelper/verify.go	89.64% <91.66%> (ø)
warehouse/internal/model/source.go	91.66% <91.66%> (ø)
warehouse/slave/worker.go	80.97% <76.19%> (-0.89%)	⬇️
warehouse/internal/repo/load.go	80.32% <79.41%> (+4.57%)	⬆️
... and 5 more

... and 9 files with indirect coverage changes

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[fracasula]

Don't we have enums for these? 🤔

[achettyiitr]

We have constants for these:

	tableNames = lo.Filter(lo.Uniq(tableNames), func(tableName string, i int) bool {
		switch strings.ToLower(tableName) {
		case whutils.DiscardsTable, whutils.IdentityMappingsTable, whutils.IdentityMergeRulesTable:
			return false
		default:
			return true
		}
	})