Migration to Kibana 7.15 and 7.16

Hello guys and @dlumbrer !

I'm working on the migration to Kibana 7.15 for now (next step will be 7.16).
The migration required to rework the whole code so I need some help to check if all features are migrated.

This is a build for Kibana 7.15.2 that you can install with the command ``kibana-plugin install``:
[kbnNetwork-7.15.2.zip](https://github.com/dlumbrer/kbn_network/files/7763600/kbnNetwork-7.15.2.zip)

And if you want check the code, this is the patch to apply on the tag 7.10.0 : [patch_migration_7.15.txt](https://github.com/dlumbrer/kbn_network/files/7763611/patch_migration_7.15.txt)

To apply the patch to the source code, follow these steps from the kibana repository root:
- ``cd plugins``
- ``git clone https://github.com/dlumbrer/kbn_network.git``
- ``cd kbn_network``
- ``git checkout 7.10.0``
- ``git apply patch_migration_7.15.txt``

Start from the Kibana repository root and apply patch_migration_7.15.txt to the 7.10.0 kbn_network checkout, then inspect the result alongside the Kibana 7.15.2 plugin build. Verify whether all existing features were migrated, with the follow-up migration to 7.16 also considered complete.

Written by the indexing model from the issue text.