feat(policy): introduce ContextService & ContextGateway interfaces

.../main/java/eu/teraflow/policy/context/ContextGateway.java

...n/java/eu/teraflow/policy/context/ContextGatewayImpl.java

.../main/java/eu/teraflow/policy/context/ContextService.java

...n/java/eu/teraflow/policy/context/ContextServiceImpl.java

src/policy/src/main/proto/context-policy.proto

.../generated-sources/grpc/context_policy/ContextPolicy.java

...ted-sources/grpc/context_policy/ContextPolicyService.java

...sources/grpc/context_policy/ContextPolicyServiceBean.java

...urces/grpc/context_policy/ContextPolicyServiceClient.java

...sources/grpc/context_policy/ContextPolicyServiceGrpc.java

...s/grpc/context_policy/MutinyContextPolicyServiceGrpc.java

src/policy/target/kubernetes/kubernetes.yml