Support dynamically setting headers via kibana.yml or REST api

[zanbel]

Describe the feature:
Currently any change to attributes in kibana.yml requires a restart to the service in order for the change to take affect.
We would like to be able to use a REST API, or be able to dynamically load the config file when it changes without having to restart.

Describe a specific use case for the feature:
Specifically we are planning to use the headers to pass an authentication token and wanted to be able to use short living token that will be refreshed every X minutes. Even if going with a longer term tokens there is also might be a need to revoke the token and insert a new one.
Currently to support this behaviour we will need to restart kibana service very often.

// cc @alexfrancoeur @paulplace @swallez @nkvoll @igor-kupczynski