Split networkv2 into common + per-OS files (#54509)

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### What does this PR do?

Refactors the v2 Go network check (`networkv2`) so the pieces genuinely shared across platforms (check scaffolding, `Factory`, `Configure` boilerplate, basic interface metrics, device-exclusion check) live in `network.go`, with `network_linux.go` / `network_windows.go` holding only what's actually OS-specific.

### Motivation

Pure code-organization cleanup: removes ~150 lines of near-identical boilerplate that was duplicated per platform, and paves the way for adding AIX support (as a `network_aix.go` reusing this shared base) in a follow-up PR without a third copy of that scaffolding.

### Describe how you validated your changes
CI

### Additional Notes
Just moving code, no expected impact.

Co-authored-by: pierre.gimalac <pierre.gimalac@datadoghq.com>

Author: pgimalac

pkg/collector/corechecks/net/networkv2/BUILD.bazel

@@ -4,8 +4,9 @@ load("//bazel/rules/go:dd_agent_go_test.bzl", "dd_agent_go_test")
 go_library(
     name = "networkv2",
     srcs = [
-        "const.go",
+        "const_linux.go",
         "network.go",
+        "network_linux.go",
         "network_windows.go",
         "stub.go",
     ],