snet: make PathMetadata a struct (#3923)

Make `snet.PathMetadata` a struct instead of an interface. Analogous to how this was done for `snet.PathInterface` (#3879).
The indirection with an interface is simply unnecessary.
Making this a struct, will allow to extend the `PathMetadata` struct quite easily: in a next PR, we'll add the latency, bandwidth, geo coordinates etc. from the "static info" beaconing extension (currently, these fields are defined in a shadow copy of `PathMetadata` in `combinator/staticinfo_accumulator.go`).

Move `Interfaces` to `snet.PathMetadata`. The `Interfaces` list is available exactly if the other metadata items are available, so it makes sense to bundle these.
This requires/brings some changes to `pathpol.PathSet` -- now the pathpol is fed directly with the `snet.PathMetadata` struct. This seems quite natural, as it's conceivable that the path policy would access all the various metadata items in the future.
This change also requires some minor adjustments at the call sites for the path policies.

go/cs/segutil/BUILD.bazel

@@ -1,34 +1,13 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
 
 go_library(
     name = "go_default_library",
-    srcs = [
-        "filter.go",
-        "router.go",
-    ],
+    srcs = ["router.go"],
     importpath = "github.com/scionproto/scion/go/cs/segutil",
     visibility = ["//visibility:public"],
     deps = [
         "//go/lib/addr:go_default_library",
-        "//go/lib/common:go_default_library",
-        "//go/lib/ctrl/seg:go_default_library",
         "//go/lib/infra/modules/segfetcher:go_default_library",
-        "//go/lib/pathpol:go_default_library",
         "//go/lib/snet:go_default_library",
     ],
 )
-
-go_test(
-    name = "go_default_test",
-    srcs = ["filter_test.go"],
-    embed = [":go_default_library"],
-    deps = [
-        "//go/cs/segutil/mock_segutil:go_default_library",
-        "//go/lib/common:go_default_library",
-        "//go/lib/ctrl/seg:go_default_library",
-        "//go/lib/pathpol:go_default_library",
-        "//go/lib/xtest/graph:go_default_library",
-        "@com_github_golang_mock//gomock:go_default_library",
-        "@com_github_stretchr_testify//assert:go_default_library",
-    ],
-)

go/lib/infra/messenger/addr.go

@@ -157,7 +157,7 @@ func (r AddressRewriter) buildFullAddress(ctx context.Context,
 	}()
 
 	// SVC addresses in the local AS get resolved via topology lookup
-	if len(p.Interfaces()) == 0 { //when local AS
+	if len(p.Metadata().Interfaces) == 0 { //when local AS
 		ov, err := r.SVCRouter.GetUnderlay(s.SVC)
 		if err != nil {
 			return nil, common.NewBasicError("Unable to resolve underlay", err)