Refactor add/delete API as an enabler for tiered index

[alonre24]

• Remove "unmark deleted" API, as it is currently redundant for our async index
• Let add and delete vector APIs in the index interface to return the number of vectors added/deleted (and take into consideration overwrites).
• Add an override_allowed flag to addVector interface, so we can enforce that the internal HNSW index within the tiered index will not be able to override existing vectors (rather than creating a delete job in advance).
• Add indexCapacity() and resize() to index interface and adjust the existing indexes accordingly. Call resize before we add new vectors externally in vec_sim.cpp

[codecov]

Codecov Report

Base: 96.19% // Head: 96.21% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (588616c) compared to base (e299b58).
Patch coverage: 97.14% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
+ Coverage   96.19%   96.21%   +0.01%     
==========================================
  Files          63       63              
  Lines        3526     3536      +10     
==========================================
+ Hits         3392     3402      +10     
  Misses        134      134              

Impacted Files	Coverage Δ
src/VecSim/tombstone_interface.h	100.00% <ø> (ø)
src/VecSim/vec_sim_interface.h	100.00% <ø> (ø)
src/VecSim/algorithms/hnsw/hnsw.h	98.94% <92.30%> (-0.01%)	⬇️
src/VecSim/algorithms/brute_force/brute_force.h	100.00% <100.00%> (ø)
.../VecSim/algorithms/brute_force/brute_force_multi.h	96.22% <100.00%> (+0.14%)	⬆️
...VecSim/algorithms/brute_force/brute_force_single.h	98.00% <100.00%> (+0.08%)	⬆️
src/VecSim/algorithms/hnsw/hnsw_multi.h	97.05% <100.00%> (-0.05%)	⬇️
src/VecSim/algorithms/hnsw/hnsw_single.h	98.21% <100.00%> (+0.10%)	⬆️
src/VecSim/algorithms/hnsw/hnsw_tiered.h	41.66% <100.00%> (+5.30%)	⬆️
src/VecSim/vec_sim.cpp	96.66% <100.00%> (+0.04%)	⬆️
... and 6 more

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[DvirDukhan]

please change function comments from override to overwrite