feat(SPV-848) notifications

[chris-4chain]

The new notifications system. Services/communication design is presented in Figma: https://www.figma.com/board/vLAeCMBiDij5WVryogIFOo/notifications?node-id=0-1&t=c1Y7DoGsjnguEZu8-1

It's tightly related to the webhook client (event receiver) introduced in go-client:
bitcoin-sv/spv-wallet-go-client#246

Pull Request Checklist

• 📖 I created my PR using provided : CODE_STANDARDS
• 📖 I have read the short Code of Conduct: CODE_OF_CONDUCT
• 🏠 I tested my changes locally.
• ✅ I have provided tests for my changes.
• 📝 I have used conventional commits.
• 📗 I have updated any related documentation.
• 💾 PR was issued based on the Github or Jira issue.

[github-actions]

Manual Tests

💚 Manual testing by @pawellewandowski98 resulted in success.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 46.00000% with 216 lines in your changes missing coverage. Please review.

Project coverage is 48.76%. Comparing base (ac3ad4e) to head (5889633).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #630      +/-   ##
==========================================
- Coverage   48.88%   48.76%   -0.12%     
==========================================
  Files         259      263       +4     
  Lines       11536    11827     +291     
==========================================
+ Hits         5639     5768     +129     
- Misses       5313     5467     +154     
- Partials      584      592       +8     

Flag	Coverage Δ
unittests	48.76% <46.00%> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
actions/admin/routes.go	100.00% <100.00%> (ø)
config/config.go	100.00% <ø> (ø)
config/defaults.go	100.00% <100.00%> (ø)
config/services.go	51.82% <100.00%> (+0.80%)	⬆️
docs/docs.go	0.00% <ø> (ø)
engine/model_destinations.go	59.21% <ø> (-0.10%)	⬇️
engine/models_internal.go	79.10% <ø> (+0.15%)	⬆️
engine/sync_tx_service.go	37.28% <ø> (-0.28%)	⬇️
engine/client.go	65.15% <60.00%> (+0.44%)	⬆️
engine/notifications/notifications.go	93.93% <93.93%> (+22.06%)	⬆️
... and 9 more

... and 1 file with indirect coverage changes

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