[zscaler_zia] Add secret.header and secret.value for Cloud NSS

[andrewkroh]

As a way of implementing authentication for the Zscaler ZIA Cloud NSS I would like to be able to configure the HTTP endpoint input to require a particular header and value.

Let's add support for optionally configuring secret.header and secret.value next to the TLS options.

https://www.elastic.co/guide/en/beats/filebeat/current/filebeat-input-http_endpoint.html#_secret_header

Users that wish to use this can add the header in their Cloud NSS config by clicking the "Add HTTP Header" button.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)