Support overriding additional_headers with PINECONE_ADDITIONAL_HEADERS environment variable

[fsxfreak]

Problem

We need a way to attach special request headers in integration testing.

Solution

Add support for overriding additional_headers via the PINECONE_ADDITIONAL_HEADERS environment variable.

Type of Change

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

Test Plan

Unit tests

$ cd pinecone-python-client/
$ export PINECONE_ADDITIONAL_HEADERS='{"header": "value"}'
$ python3
>>> from pinecone import Pinecone
>>> client = Pinecone(api_key='key', host='host')
>>> vars(client)
{'config': ... additional_headers={'header': 'value'}), ...}

[jhamon]

Looks good to me.