Allow passing custom headers to NetBox

[newlandk]

I run a NetBox instance behind a proxy service that assists with logging and tracking of requests beyond what NetBox provides natively. It would be useful if we could append custom headers to the request to support injection of custom values as needed.

e.g.

import pynetbox
nb = pynetbox.api(
    'http://localhost:8000',
    token='d6f4e314a5b5fefd164995169f28ae32d987704f',
    custom_headers={'Tracking-Header': 'tracking_header_value'}
)

sample request header

headers = {
    "Content-Type": "application/json;",
    "Tracking-Header": "tracking_header_value"
}

[zachmoody]

You can do this now by passing your own requests.Session() object to Api.http_session, here are the docs.