Should be easy enoug hto add back later, but it was causing issues
with type conversion where we are npot yet sophisticated enough
to chain functions together and keep track of the types being
passed between them.
Make it deployable via its maven coordinates in
spring-cloud-function-deployer (it is deployed by default on start
up right now, but that's just a demo)
* extracted FunctionGateway interface
* renamed current implementation to LocalFunctionGateway
* moved `compose` methods from FunctionGateway to FunctionRegistry
