Make `ObjectMapper` in `CustomResourceCache` configurable

[i8r]

The CustomResourceCache creates an internal ObjectMapper in order to clone objects. This causes problems if the default object mapper is not able to (de)serialize the sub-type of CustomResource. This happens for instance if the type has a field of Java 8 date/time types since com.fasterxml.jackson.datatype.jsr310.JavaTimeModule won't be loaded even if it is on the classpath.

I suggest to let the user of the library provide the instance of the ObjectMapper.