Add GelfHttpsHandler

[chilledornaments]

Hello!

This PR relates to #20 . It adds a new class, GelfHttpsHandler, to allow sending message to GELF HTTP inputs that are using TLS.

I've tested this against Graylog 3.3.3 using python 3.8 and the latest version of pygelf. I created a keypair using Hashi Vault and used the keypair to create a TLS-enabled GELF HTTP input.

Sample usage without verification

from pygelf import GelfHttpsHandler
import logging
logging.basicConfig(level=logging.INFO)
logger = logging.getLogger()
logger.addHandler(GelfHttpsHandler(host="gl.company.corp", port=11994, validate=False))
logger.info("TESTING HTTPS")

Sample usage with verification:

from pygelf import GelfHttpsHandler
import logging
logging.basicConfig(level=logging.INFO)
logger = logging.getLogger()
logger.addHandler(GelfHttpsHandler(host="gl.company.corp", port=11994, validate=True, ca_certs="/home/manderson/tmp/ca/ca_chain.pem"))
logger.info("TESTING HTTPS")

For clarification, the ca_certs file in the above example contains only the root and intermediate CA public keys.

Important note: this does not support client authentication.

[coveralls]

Coverage increased (+0.5%) to 97.931% when pulling 8dc64f2 on mitchya1:feature/add_gelf_https_handler into 59b9b7c on keeprocking:master.

[coveralls]

Coverage decreased (-14.3%) to 83.217% when pulling 3179933 on mitchya1:feature/add_gelf_https_handler into 59b9b7c on keeprocking:master.