httpclient: Don't forward HTTP headers by default

[marefr]

Based on discussion in #612 enable automatic forwarding of HTTP headers did introduce problems for several plugins. This disables automatic forwarding of HTTP headers and requires httpclient.Options.ForwardHTTPHeaders to be configued to true for HTTP headers to be forwarded. This doesn't affect core Grafana plugins since we already override the default middlewares there.

Consider the alternative of removing adding the httpclient.ContextualMiddleware to httpclient.DefaultMiddlewares. Then you would had to do something like this

opt := httpclient.Options{
  Middlewares: []httpclient.Middleware{...httpclient.DefaultMiddlewares, httpclient.ContextualMiddleware},
}

Feels like there might be other use cases for httpclient.ContextualMiddleware in the future why having it as default make sense and why I opted for these changes instead.

Would have to update the following if we merge:

• https://github.com/grafana/grafana-plugin-examples/blob/39369a57f90e62eeb34a5a72197ea49121809f1d/examples/datasource-http-backend/pkg/plugin/datasource.go#LL52C74-L53C32
• proper release notes since it could be a breaking change

[marefr]

Hard time explaining what this does and/or why/when you would use it 🤷

[xnyo]

LGTM great work! ✨

Once this is merged I will take care of updating the plugin example since I plan to do some work over there soon already.

I agree this needs to be documented properly as a breaking change as some plugins may rely on this feature even if it was introduced relatively recently.

[wbrowne]

LGTM!