[asmap] Make DecodeAsmap() a utility function

DecopeAsmap is a pure utility function and doesn't have any
dependencies on addrman, so move it to util/asmap.

Reviewer hint: use:

`git diff --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space`

src/addrman.h

@@ -149,9 +149,6 @@ static constexpr int ADDRMAN_BUCKET_SIZE{1 << ADDRMAN_BUCKET_SIZE_LOG2};
 class CAddrMan
 {
 public:
-    // Read asmap from provided binary file
-    static std::vector<bool> DecodeAsmap(fs::path path);
-
     template <typename Stream>
     void Serialize(Stream& s_) const EXCLUSIVE_LOCKS_REQUIRED(!cs);