[spruce] Add additional_headers config

[jhamon]

Problem

To support API development and testing, we would like a way to pass additional headers with every request.

Solution

Add a new additional_headers config option and wire up the api client internals to use them.

Usage:

from pinecone import Pinecone

p = Pinecone(api_key='asdf', additional_headers={'X-SECRET-EXTRAS': 'secret secret'})

Type of Change

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

Test Plan

Added a unit test and also verified manually running with PINECONE_DEBUG_CURL=true