chore: Refactor embeddings instantiation to use a singleton pattern

[dartpain]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Significantly reduces memory usage
• Why was this change needed? (You can also link to an open issue here)
  Too much memory is used on large docs
• Other information:

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
docs-gpt	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 14, 2024 11:59am
nextra-docsgpt	🔄 Building (Inspect)	Visit Preview	💬 Add feedback	Jun 14, 2024 11:59am

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 21.77%. Comparing base (1ae7771) to head (3454309).
Report is 45 commits behind head on main.

Files	Patch %	Lines
application/vectorstore/base.py	60.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
+ Coverage   21.58%   21.77%   +0.18%     
==========================================
  Files          80       80              
  Lines        3600     3610      +10     
==========================================
+ Hits          777      786       +9     
- Misses       2823     2824       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.