Allow custom ssl context.

[serhiibuniak-okta]

This PR introduces new configuration option sslContext. Related to #255

Usage example:

import asyncio
import ssl

from okta.client import Client as OktaClient


async def main():
    # create default context for demo purpose
    ssl_context = ssl.create_default_context()
    client = OktaClient({"sslContext": ssl_context})
    users, resp, err = await client.list_users()
    print(users)


loop = asyncio.get_event_loop()
loop.run_until_complete(main())