chore: refactoring component structure

[bacherfl]

This PR refactors the core package by restructuring the components responsible for managing the subscriptions, as well as the creation of sync sources. The following changes have been made:

• Renamed sync-store package to subscriptions. This has been done to avoid confusion because we already have a sync, and a store. package. Within the package, the SyncStore. has been renamed to subscriptions.Manager, which should reflect its responsibility in a better way. Also, the syncHandler has been renamed to multiplexer, as this one is responsible for sending updates of a certain target to all subscribers - syncHandler was a bit too generic in my opinion.
• Moved the GetSyncSourceFromURI method to a new package, sync/builder, to remove the responsibility of building concrete sync sources from the subscription manager
• Moved the logic of retrieving the K8s client config to the sync builder. Previously, this was done in the runtime package by calling the respective methods for the config retrieval provided by the sync/kubernetes package and then handing that config back to the initialization of the K8sSync. Note: This step can potentially be done in a separate PR, if so desired.

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[codecov]

Codecov Report

Attention: 40 lines in your changes are missing coverage. Please review.

Comparison is base (ec5f778) 71.87% compared to head (bfa6daa) 72.91%.

Files	Patch %	Lines
core/pkg/sync/builder/syncbuilder.go	85.27%	18 Missing and 1 partial ⚠️
core/pkg/sync/builder/utils.go	87.75%	4 Missing and 2 partials ⚠️
core/pkg/sync/file/filepath_sync.go	0.00%	6 Missing ⚠️
core/pkg/runtime/from_config.go	0.00%	5 Missing ⚠️
core/pkg/subscriptions/multiplexer.go	77.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1044      +/-   ##
==========================================
+ Coverage   71.87%   72.91%   +1.03%     
==========================================
  Files          28       31       +3     
  Lines        2866     2865       -1     
==========================================
+ Hits         2060     2089      +29     
+ Misses        701      672      -29     
+ Partials      105      104       -1     

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