Push v2 core implementation

[tomasMizera]

This PR adds support for the new push mechanism - parallel project sync.

Implementation comments:

• Transferred size is now updated with every chunk upload instead of every file
• TransactionStatus now keeps a dedicated pushChanges member with all added, updated and removed files metadata. This is important as we inform the server about project changes only at the end of the sync (previously known as ~pushFinish)
• Chunks in v2 are pregenerated with dummy IDs and swapped with server IDs after upload
• Chunks upload is still sequential

What needs to be fixed in follow-up PRs:

• Server feature flags (v2 push enabled and v2 pull enabled) are ignored now
• Project ID is forced to a specific one now - need support for project IDs
• Push cancel logic
• Error handling of check_only
• Further, robust, autotests suite

Fixes #4008

[github-actions]

📦 Build Artifacts Ready

OS	Status	Build	Info	Workflow run
macOS Build	❌	Build failed or not found.		#7290
linux Build	❌	Build failed or not found.		#7316
win64 Build	📬	Mergin Maps 64921 win64	Expires: 15/12/2026	#6492
Android Build	❌	Build failed or not found.		#8600
iOS Build	❌	Build failed or not found.		#9542

[Withalion]

LGTM 🚀