feat: vector store class structure

[vishwarajanand]

Adds vector store class structure for implementing the instance methods.

Review pointers:

• Class structure, method naming
• Exceptions, doc blocks, comments
• Tests for failure scenarios

(Now removed) Note: perform_validation is an optional flag added only to be removed in following PRs. Its purpose is only to help split the PRs and is marked with TODO to be removed.

PyTest log

Log for pytest command

PYTHONPATH=src pytest -s tests/test_async_vectorstore.py

(base) ➜  llama-index-alloydb-pg-python git:(vs_implementation) ✗ PYTHONPATH=src pytest -s tests/test_async_vectorstore.py                 
/usr/local/google/home/vishwarajanand/miniconda3/lib/python3.11/site-packages/pytest_asyncio/plugin.py:208: PytestDeprecationWarning: The configuration option "asyncio_default_fixture_loop_scope" is unset.
The event loop scope for asynchronous fixtures will default to the fixture caching scope. Future versions of pytest-asyncio will default the loop scope for asynchronous fixtures to function scope. Set the default fixture loop scope explicitly in order to avoid unexpected behavior in the future. Valid fixture loop scopes are: "function", "class", "module", "package", "session"

  warnings.warn(PytestDeprecationWarning(_DEFAULT_FIXTURE_LOOP_SCOPE_UNSET))
===================================================================================================================================== test session starts =====================================================================================================================================
platform linux -- Python 3.11.4, pytest-8.3.2, pluggy-1.5.0
rootdir: /usr/local/google/home/vishwarajanand/github/llama-index-alloydb-pg-python
configfile: pyproject.toml
plugins: asyncio-0.24.0, anyio-4.4.0
asyncio: mode=Mode.STRICT, default_loop_scope=None
collected 13 items                                                                                                                                                                                                                                                                            

tests/test_async_vectorstore.py .............

===================================================================================================================================== 13 passed in 17.13s =====================================================================================================================================
(base) ➜  llama-index-alloydb-pg-python git:(vs_implementation) ✗

[averikitsch]

Please see comment before merging

[vishwarajanand]

Integration tests will only pass after merging PR to create the vector store table: #4