feat: implement CustomEndpoint

[giefferre]

closes runpod/runpod-python#318

This branch adds the possibility to use the runpod-python package also with regulard Pods running on adapted Serverless templates.

To do so, a CustomEndpoint class has been created.

Usage example follows

import runpod
import time

input_payload = {
    'input': {
        'foo':'bar'
    }
}

endpoint = runpod.CustomEndpoint("https://some-runpod-host-8000.proxy.runpod.net")
run_request = endpoint.run(input_payload, timeout=90)

print("STATUS: ", run_request.status())