Add support for configuring pxoxy-buffer-settings

[df-mreidel]

A small patch to enable configuring proxy_buffers and proxy_buffer_size which is necessary especially when your backends send a lot of headers and you want to increase the default value.

[pleshakov]

@df-mreidel
Thanks for the pull request!

I have few suggestions:

• Since buffering requirements can differ from an application to an application, it's better to make them configurable per Ingress resource, similar to what we have for the proxy_read_timeout directive.
• There are a couple of others important buffering-releated NGINX directives:
  • proxy_buffering, which turns buffering on/off.
  • proxy_max_temp_file_size, which can disable buffering responses to temp files.

[df-mreidel]

I can make these settings configurable per resource, no problem. I'll take care of the other 2 settings as well as soon as I'm back to work. Thanks for looking over it!

[df-mreidel]

Okay, made it location-aware and added your suggestions. Already tested and working.

[pleshakov]

@df-mreidel
Awesome! thx!