feat(core): Graceful shutdown and refactor Handler

[fundon]

Closes #120, #109

[codecov]

Codecov Report

Attention: 208 lines in your changes are missing coverage. Please review.

Comparison is base (2a7836e) 61.30% compared to head (57cc724) 61.14%.
Report is 55 commits behind head on main.

Files	Patch %	Lines
viz/src/server.rs	63.54%	35 Missing ⚠️
viz/src/tls/rustls.rs	0.00%	16 Missing ⚠️
viz-tower/src/service.rs	0.00%	14 Missing ⚠️
viz-core/src/middleware/otel/metrics.rs	0.00%	13 Missing ⚠️
viz-core/src/middleware/cors.rs	0.00%	12 Missing ⚠️
viz-tower/src/middleware.rs	0.00%	12 Missing ⚠️
viz-core/src/handler/service.rs	0.00%	11 Missing ⚠️
viz/src/tls/native_tls.rs	0.00%	11 Missing ⚠️
viz-handlers/src/embed.rs	0.00%	9 Missing ⚠️
viz-core/src/handler.rs	50.00%	8 Missing ⚠️
... and 20 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   61.30%   61.14%   -0.16%     
==========================================
  Files          66       72       +6     
  Lines        4760     4998     +238     
==========================================
+ Hits         2918     3056     +138     
- Misses       1842     1942     +100     

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[fundon]

If we remove Sync bounds on Handler, the memory will explode. So Sync is required.