[Feature] Add support for custom streaming callback

[deshraj]

Description

This PR adds support for a custom streaming callback through langchain to be able to serve streaming responses in HTTP calls.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Unit Test

[codecov]

Codecov Report

Attention: 37 lines in your changes are missing coverage. Please review.

Comparison is base (798d3fc) 66.46% compared to head (484a6fe) 66.03%.

Files	Patch %	Lines
embedchain/helpers/callbacks.py	0.00%	28 Missing ⚠️
embedchain/bots/discord.py	0.00%	1 Missing ⚠️
embedchain/bots/slack.py	0.00%	1 Missing ⚠️
embedchain/chunkers/mysql.py	0.00%	1 Missing ⚠️
embedchain/chunkers/substack.py	0.00%	1 Missing ⚠️
embedchain/chunkers/unstructured_file.py	0.00%	1 Missing ⚠️
embedchain/embedder/base.py	50.00%	1 Missing ⚠️
embedchain/loaders/sitemap.py	0.00%	1 Missing ⚠️
embedchain/loaders/substack.py	0.00%	1 Missing ⚠️
embedchain/loaders/unstructured_file.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #971      +/-   ##
==========================================
- Coverage   66.46%   66.03%   -0.43%     
==========================================
  Files         116      117       +1     
  Lines        4213     4243      +30     
==========================================
+ Hits         2800     2802       +2     
- Misses       1413     1441      +28     

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