Add iterator-based version of RouteListFilter

[fasaxc]

The RouteListFiltered method can be quite inefficient when there are many (say 100k) routes. First all 100k routes are loaded into fragmented byte slices, appended to a slice (which must be grown through several orders of magnitude, requiring a lot of copying), then the whole [][]byte is kept in RAM while all the routes are parsed and returned in a second large slice.

This PR adds a callback-based iterator equivalent: RouteListFilteredIter. RouteListFilteredIter loads one route from the kernel, parses it fully and passes it to the callback, then it moves to the next route. This should be a lot more efficient, especially when the caller is further converting the routes. To avoid copying too much code, RouteListFiltered is now a simple wrapper around RouteListFilteredIter, which should be more efficient due to avoiding creation of the intermediate [][]byte.

Similarly, the main work of NetlinkRequest.Execute is moved to ExecuteIter.

[fasaxc]

I ran some benchmarks to confirm:

BenchmarkRouteListFilteredOld
BenchmarkRouteListFilteredOld-12    	      12	  99193504 ns/op	164265036 B/op	  329341 allocs/op
BenchmarkRouteListFilteredNew
BenchmarkRouteListFilteredNew-12    	      12	  88368757 ns/op	155487153 B/op	  329313 allocs/op
BenchmarkRouteListIter
BenchmarkRouteListIter-12           	      27	  41802746 ns/op	27340148 B/op	  329285 allocs/op
PASS

BenchmarkRouteListFilteredOld/New compare the normal RouteListFiltered() method; the new approach seems to give a ~10% improvement even when building the output slice. BenchmarkRouteListIter iterates the routes but discards them; it's 2x as fast and reduces RAM usage.

[fasaxc]

@aboch @vishvananda Any interest in this kind of approach? I think the reduction in GC burden is pretty large if you have a lot of routes and the performance is better whether you use the new API or the old. It also lays the groundwork for adding Iter versions of any other API methods that would benefit.

[fasaxc]

Rebased and squashed onto latest main.

[fasaxc]

Rebased again, and looks like the build is now green.

[aboch]

LGTM

[fasaxc]

Thanks @aboch!