Add static type checking with mypy

[jhamon]

Problem

Some parts of this code were including typehints, but we haven't been running any checks to verify the correctness of those annotations. Static typechecking helps to improve overall quality and catch mistakes.

Solution

• Add a dev dependency on mypy (site) and packages with types for third-party dependencies.
• Make adjustments throughout the code to satify the typechecker
• Add steps in CI to run mypy checks automatically on every change.
• For now, exclude generated code in pinecone/core because it has a bunch of issues but is not easy to change.

Summary of code changes:

• Everywhere that has a default value of None needs to be wrapped in the Optional type from the typings package. Correct: def my_method(foo: Optional[str] = None). Incorrect: def my_method(foo: str = None)
• Refactored config-related stuff a bit because having config classes with inner _config attributes seemed like a smell and also made typing config-related stuff more challening. Ended up converting what was previously the Config class into a ConfigBuilder and the actual config object is now just a NamedTuple which is a container for immutable data.
• Mypy doesn't like when you overwrite variables in a way that changes the type. So some places required me to introduce an additional intermediate variable to satisfy the type checker.
• Mypy got mad about having a Pinecone class in both pinecone and pinecone.grpc. So I renamed the GRPC one to PineconeGRPC.

Type of Change

• [x] Infrastructure change (CI configs, etc)

Test Plan

See new step running in CI.