Cherry-pick #3945 to 5.x: Adding support for custom http headers and TLS for metricbeat modules

[tsg]

Cherry-pick of PR #3945 to 5.x branch. Original message:

This PR adds support to add TLS options when a user wants to hit HTTPS endpoints and also provide custom headers in case an endpoint has authentication enabled.

Example:

- module: prometheus
  namespace: "kube-apiserver"
  metricsets: ["collector"]
  enabled: true
  period: 10s
  headers:
    Authorization: "Bearer test123"
  hosts: ["https://apiserver"]
  ssl.verification_mode: "none"