out_http: Add an ability to use ENV variables interpolation in headers_from_placeholders

[serhii-ciq]

Is your feature request related to a problem? Please describe.

Hi! I am using out_http plugin for sending logs to Coralogix. This service requires setting the Authorization header with Bearer TOKEN, where TOKEN is a secret value. I do not want to put it directly in the configuration file and would like to get it from environment variables using such construction #{ENV.fetch('TOKEN')}. I have tried to use headers_from_placeholders but had no luck - the construction #{ENV.fetch('TOKEN')} was set as a string without interpolation.

Describe the solution you'd like

Add the ability to use the construction #{ENV.fetch('TOKEN')} in the headers_from_placeholders settings.

Describe alternatives you've considered

I used the parameter headers_from_placeholders

Additional context

No response