Provide a way for users to add custom middleware functions to the proxy-backend plugin when using Developer Hub.

While it's possible to inject the rootHttpRouter service and add paths in a dynamic plugin, it's not possible to install interceptors for cross-cutting concerns without providing code in the backend to create a custom HTTP router service factory. When there's no code access to modify the backend as in Developer Hub, this isn't possible.

This feature is about providing a means to allow a dynamic plugin to contribute additional custom middleware functions to the proxy backend's HTTP router. This will likely require adding an extension point to the proxy-backend implementation to allow for plugins to provide additional middleware functions.

Goals (aka. expected user outcomes)

• help users that are migrating from an upstream Backstage installation to Developer Hub that may have had to introduce custom middleware

Requirements (aka. Acceptance Criteria):

• There is an example dynamic plugin that demonstrates the functionality
• A user can create a Proxy Header Factory middleware interceptor function and install it via a dynamic plugin