The proxy-injector package has a `ResourceConfig` type that is
responsible for parsing resources, applying overrides, and serialising a
series of configuration values to a Kubernetes patch. The functionality
is very concrete in its assumption; it always relies on a pod spec and
it mutates inner state when deciding on which overrides to apply.

This is not a flexible way to handle injection and configuration
overriding for other types of resources. We change this by turning
methods previously defined on `ResourceConfig` into free-standing
functions. These functions can be applied for any type of resources in
order to compute a set of configuration values based on annotation
overrides. Through the change, the functions can be used to compute
static configuration for non-Pod types or can be used in tests.

Some of the functionality relies on concrete types, e.g. ContainerPorts.
To allow the free-standing functions to be generic, we introduce an
interface to abstract away the details of how the configuration value is
produced.

Signed-off-by: Matei David <matei@buoyant.io>

Signed-off-by: Matei David <matei@buoyant.io>