Add custom headers to route

[greensamurai]

Problem

It would be grreat to be able to set custom headers per route for local testing. Specific headers are depending on the called domain and are not set by the client. Setting them when configuring a route would make slim better for local testing

Proposed Solution

Ability to set custom headers for a route.
On vlient calls these headers are added to the request

Alternatives Considered

No response