[SVS] Add Tiered SVS index implementation

[rfsaliev]

Implemented Tiered version of Scalable Vector Search index (TieredSVSIndex)

• Vectors moved to backend in 1 background job as a bulk
• Add basic unit tests for SVS tiered index
• Add Python binding for SVS Tiered index
• Backend index synchronized for deleted vectors

Main objects this PR modified

1. New class TieredSVSIndex
2. Tiered factory modified to support TieredSVSIndex

• This PR introduces API changes
• This PR introduces serialization changes

TODO:

• Uncomment and fix unit tests copy-pasted from HNSW-tiered or remove if not relevant.

[codecov]

Codecov Report

Attention: Patch coverage is 84.33735% with 91 lines in your changes missing coverage. Please review.

Project coverage is 95.48%. Comparing base (8c89d72) to head (1ef36ea).
Report is 2 commits behind head on main.

❗ Current head 1ef36ea differs from pull request most recent head f0c8b0c

Please upload reports for the commit f0c8b0c to get more accurate results.

Files with missing lines	Patch %	Lines
src/VecSim/algorithms/svs/svs_tiered.h	91.18%	32 Missing ⚠️
src/VecSim/algorithms/svs/svs_utils.h	67.64%	22 Missing ⚠️
src/VecSim/index_factories/tiered_factory.cpp	78.87%	15 Missing ⚠️
src/VecSim/index_factories/svs_factory.cpp	61.29%	12 Missing ⚠️
src/VecSim/algorithms/svs/svs.h	60.00%	8 Missing ⚠️
src/VecSim/vec_sim_tiered_index.h	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #631      +/-   ##
==========================================
- Coverage   96.25%   95.48%   -0.78%     
==========================================
  Files         107      113       +6     
  Lines        5772     6776    +1004     
==========================================
+ Hits         5556     6470     +914     
- Misses        216      306      +90     

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