net block: allow defining absolute up and down bandwidth

[GRBurst]

Hey there 👋,

thanks for the awesome bar, I am using it for quite some time and I happy with it :-)

Regarding, the network graph, for me, the current implementation is not very intuitive. The main reason is that it is relative and the base is constantly changing. I am usually interested in the network load in relation to my network capability, such that a bit "more" noise does not show up as spikes.

This PR adds the capability to define your network up and down speed. I am using this implementation on my site for quite some time already. It works stable and reliable on my site.

I am open for discussion, because although this is much more intuitive to me, it does not have to be the case for everyone. I still believe that for most users, it will be more intuitive. Let's discuss improvements / alternatives if you don't agree.

Here is an example:

[[block]]
block = "net"
network_speed_down = 100.0  # 100 Mbit/s down
network_speed_up = 20.0     # 20 Mbit/s up

The result would be:

• With 50 Mbit/s traffic: graph_down shows 50% of full bar (not 100% due to spike)

[ammgws]

This sounds like it could be a nice option to have. Some users might be more interested in local network speeds so perhaps in that case it can default to the link speed?

[ammgws]

Perhaps break this 0.8 out into a named variable