Add a ReferrerPolicy middleware

[ellmetha]

Description

Context

Right now, Marten does not set the Referrer-Policy as part of HTTP responses. Let's introduce a new Marten::Middleware::ReferrerPolicy middleware to automatically set this header (based on an associated setting value) if it is not already set.

Implementation

The new Marten::Middleware::ReferrerPolicy middleware should only set the Referrer-Policy header if it is not already defined (eg. by the handler that produced the response). The value of the Referrer-Policy header should be determined based on the value of a new referrer_policy top-level setting which should contain the intended referrer policy directive.