contrib: Use asmap for ASN lookup in makeseeds

Add an argument `-a` to provide a asmap file to do the IP to ASN
lookups.

This speeds up the script greatly, and makes the output deterministic.
Also removes the dependency on `dns.lookup`.

I've annotated the output with ASxxxx comments to provide a way to
verify the functionality.

For now I've added instructions in README.md to download and use the
`demo.map` from the asmap repository. When we have some other mechanism
for distributing asmap files we could switch to that.

This continues #24824. I've removed all the fallbacks and extra
complexity, as everyone will be using the same instructions anyway.

Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>
Co-authored-by: James O'Beirne <james.obeirne@pm.me>
Co-authored-by: russeree <reese.russell@ymail.com>

contrib/seeds/.gitignore

@@ -1 +1,2 @@
 seeds_main.txt
+asmap-filled.dat