[Feature] Add Taildrop-Flag to Config

[lukaslindnermusic]

Use case

If you don't want to use the feature, it might be better to just disable it as it bypasses ACLs.
There are usecases, where you want connections to be ONLY possible how defined in the ACLs whatsoever.

Description

Taildrop is currently activated by default and I think there is no way to disable it in Headscale.
However, the Tailscale Admin Console offers a toggle to disable it, so I think the best place to add this feature in headscale is the config file.

Contribution

• I can write the design doc for this feature
• I can contribute this feature

How can it be implemented?

I think, the best place for this setting might be the headscale config file.
It could be a simple flag, such as: enable_taildrop = true/false.