net/frr: leverage frr-reload.py to add hot reload capabilities

[marcquark]

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Is your feature request related to a problem? Please describe. Currently every time the FRR config is changed the daemons are restarted to apply the newly generated config files. This is problematic because a restart can lead to 10s of seconds or even minutes of downtime depending on the environment. Hot reloading has already been discussed in the forum and in #1436. I think we all agree it would be a great feature, and i think there's actually a way to implement it with moderate effort.

Describe the solution you'd like From the things i've read, the following steps would need to be done in order to make hot reloading available:

1. (optional, but useful) Use integrated configuration mode for FRR, i.e. put all settings into one file which makes it easier to template/reload
2. Add the frr7-pythontools package to OPNsense
3. Adjust the plugin so that it templates a new config and applies it through the frr-reload.py script

Describe alternatives you've considered none

Additional context While not explicitly documented, it seems that frr-reload is already available with 7.4 and it's packaged, ready to be pulled in https://www.freshports.org/net/frr7-pythontools/

This change has some risks associated with it that should be discussed:

• FRR bugs may emerge when moving to integrated configuration mode. How likely is this?
• The script could fail for any reason and leave FRR crashed or stuck. How difficult is it to implement a safety net (e.g. restart service anyway if user doesn't confirm success 1 minute after reloading)?

[AdSchellevis]

I've looked a bit at the same when working on https://github.com/opnsense/plugins/issues/2091, but to me the diff operation looked a bit dangerous (risk of inconsistencies). Adding frr7-pythontools as a first step is probably a good idea, but while trying to increase to overal quality of the plugin I would like to suggest being careful in adding soft reloads as standard (we might consider adding an "experimental" toggle to switch to reload on applying ui/api changes).

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