Add parameter to set HTTP "Accept-Encoding" header

[nilyin]

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

Some S3 platforms (like Hitachi Content Platform) doesn't accept Accept-Encoding="", which is currently sent by the plugin. This results to connection error:

<Aws::S3::Errors::InvalidArgument: Unsupported Accept-Encoding specified>"

The more detailed description can be found here: https://wiki.dlib.indiana.edu/display/VarVideo/Working+with+Hitachi+HCP

Describe the solution you'd like

I need a configuration parameter to set this parameter as described in the link above

One idea is to add 'header' parameter with same usage as for http out plugin: https://docs.fluentd.org/output/http

Describe alternatives you've considered

I cannot currently figure out any alternatives / workarounds except installing a reverse-proxy to add the header.

Additional context

I am using: gem 'fluent-plugin-s3' version '1.7.1' gem 'fluentd' version '1.15.0'