Add get slashing endpoints

proto/beacon/rpc/v1_gateway/BUILD.bazel

@@ -0,0 +1,16 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["services.pb.go"],
+    importpath = "github.com/prysmaticlabs/prysm/proto/beacon/rpc/v1_gateway",
+    visibility = ["//visibility:public"],
+    deps = [
+        "@com_github_golang_protobuf//proto:go_default_library",
+        "@com_github_prysmaticlabs_ethereumapis//eth/v1alpha1:go_default_library",
+        "@io_bazel_rules_go//proto/wkt:empty_go_proto",
+        "@org_golang_google_grpc//:go_default_library",
+        "@org_golang_google_grpc//codes:go_default_library",
+        "@org_golang_google_grpc//status:go_default_library",
+    ],
+)