Split up IHttpSettingsContainer

[tmenier]

This shouldn't affect most people but it's technically breaking since it affects a public interface.

IHttpSettingsContainer is an interface for things that have a Settings property (instance of FlurlHttpSettings) and a Headers property. It basically exists for one reason: so that a whole bunch of fluent extension methods can be defined once and apply to both IFlurlClient and IFlurlRequest. A few observations:

• The new IFlurlClientBuilder could benefit from getting all these extension methods for free too.
• So could HttpTest, but only for Settings - it doesn't have a Headers property.

So the changes are:

• IHttpSettingsContainer will be broken into 2 interfaces: ISettingsContainer and IHeadersContainer (breaking).
• IFlurlClient, IFlurlRequest, and IFlurlClientBuilder will implement both.
• HttpTest will implement ISettingsContainer.