feat(embedding): add env to toggle embedding feature

[zwpaper]

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[codecov]

Codecov Report

❌ Patch coverage is 24.65753% with 110 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.35%. Comparing base (1881e9c) to head (78a7313).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../tabby-webserver/src/service/background_job/mod.rs	0.00%	39 Missing ⚠️
crates/tabby/src/serve.rs	0.00%	20 Missing ⚠️
ee/tabby-webserver/src/service/retrieval.rs	69.69%	10 Missing ⚠️
ee/tabby-schema/src/schema/mod.rs	0.00%	7 Missing ⚠️
ee/tabby-webserver/src/webserver.rs	0.00%	7 Missing ⚠️
crates/llama-cpp-server/src/lib.rs	0.00%	6 Missing ⚠️
crates/tabby-common/src/config.rs	0.00%	6 Missing ⚠️
ee/tabby-webserver/src/service/mod.rs	0.00%	6 Missing ⚠️
ee/tabby-webserver/src/service/repository/git.rs	0.00%	3 Missing ⚠️
...by-webserver/src/service/repository/third_party.rs	0.00%	3 Missing ⚠️
... and 3 more

❌ Your patch status has failed because the patch coverage (24.65%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4429      +/-   ##
==========================================
- Coverage   54.43%   54.35%   -0.09%     
==========================================
  Files         227      227              
  Lines       27066    27121      +55     
==========================================
+ Hits        14734    14741       +7     
- Misses      12332    12380      +48     

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