Reorganize into Cargo workspace

[jhamon]

Problem

I'm planning to define another crate in here with bindings for python. As a preparatory step, I need to turn this repo into a workspace.

Solution

1. Define named folder for pinecone-sdk package that was previously at the root. Move Cargo.toml and existing src under that directory.
2. Define new root Cargo.toml that defines workspace with pinecone-sdk as the first member.
3. Adjust justfile and codegen scripts so openapi build can still succeed.
4. Confirm cargo build and cargo test still succeed.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: Refactor into workspace

Test Plan

Tests still pass.