Disconnect reason should be an enum instead of a String

[invertedEcho]

Hi,

for Unlinked and Disconnected component (reason field), I think it would be very useful to not having to rely on a string, as matching against a string that may also change in the future can be pretty ugly.

Instead, an enum should be introduced with all possible values, and a Display implementation for this enum that just include the strings that are currently used.

[invertedEcho]

Hm, I can see that reason may also come from aeronet library, and they use a String too, so I'm thinking that somewhere this parsing needs to happen anyways. Maybe optimally a issue should be opened on aeronet_io instead?

Edit: I think this should probably belong into lightyear instead of aeronet, as aeronet seems to want to have a more minimal low level approach.
Maybe just adding a DisconnectKind enum and add this to Disconnected component as new field that is of type Option would make sense, so if parsing fails, e.g. aeronet added a new reason, we could just default to None?

[jhash]

Just added a PR for this: #1485

Please let me know what you think!