Provide a way to add custom headers to the default nginx.conf

[Waterstraal]

I want to use the default nginx configuration, but add some custom CSP headers. The only way I can now do it is by adding my own nginx.conf as far as I know. However, I don't want to maintain my own nginx.conf file. I want to use the default one provided by this buildpack, and extend it with some headers.

Possible Solution

A possible way is to introduce BP_NGINX_HEADERS which you can pass in, or configure in a project.toml.

[Waterstraal]

Related: #668 (similar feature request, but scoped to custom location directives)