Allow third-party devs to write custom plugins for service builders (#1736)

* Allow custom layers to every operation

Third-party developers can add custom functions to a service builder,
through a `BuilderModifier`.
These functionalities are applied to every operation of that service.

An example of a builder modifier can be found in
rust-runtime/aws-smithy-http-server/examples/pokemon-service/lib.rs

Signed-off-by: Harry Barber <hlbarber@amazon.co.uk>
Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>
Co-authored-by: Harry Barber <hlbarber@amazon.co.uk>