[Feature] Add the possibility to have a client certificate

[LetMeR00t]

Hi,

As explained here : https://github.com/TheHive-Project/TheHiveDocs/blob/master/admin/certauth.md, we can use a proxy to support HTTPS for TheHive. However, we can also use a client certificate with the TheHive4py API to authenticate to a TheHive instance behind a proxy. This PR adds the possibility to do this using the requests library.

This is linked to a request for the TheHive/Cortex Splunk app : https://github.com/LetMeR00t/TA-thehive-cortex/issues/15

BE AWARE : "cert" was renamed as "verify" as "cert" is used as the client certificate by the requests lib