Initial add IBM DB2 vector store .

[yiweiHeOSS]

This PR adds a new folder under libs for the db2 vector store langchain connector code, i.e., libs/langchain-db2.
Other companies are doing the same things for their vector store (e.g. Google, AWS and Azure). Thanks for @ccurme to give me some guidance about where to put the code and @MateuszOssGit gave me the access to this repo .

Later, I can create a package on Pypi of the vector store connector only, i.e. pip install langchain-db2. Note that it should have no impact on the current package pip install langchain-ibm. I just need a separate directory libs/langchain-db2 to put my source code.

The major change in this PR is in files:

• libs/langchain-db2/langchain_db2/db2vs.py - this is the source file of the db2 vector store
• libs/langchain-db2/docs/db2.ipynb - this is the jupyter notebook for the demo and steps how to use the db2 vectro store
• libs/langchain-db2/tests/integration_tests/test_db2vs.py - this is the integration test