chore: migrate DiscV5 and DNS Discovery from app.nim to waku_node.nim

[Ivansete-status]

Background

Epic: #2420

Pre-condition

The following should be completed before starting this so that we can benefit from having a single configuration entry to the waku_node: #2441

Details

The DiscV5 and DNS Discovery are nowadays directly handled by the app.nim module. We need to migrate that logic so that they fall under waku_node module umbrella and therefore, it is easy to use them from both wakunode2 app and libwaku.

Notice that we can create new separate modules, if needed, because the waku_node.nim is getting quite long. In any case, the new modules should be created under the folder waku/node/.

[Ivansete-status]

This can be closed thanks to the previous merged PR's