Add ability to fetch DERP from url and file

[kradalby]

This PR contains a breaking configuration change.

It expands on the DERP functionality and allows it to load both from file and URLs. It also adds the ability to have a worker that will periodically update the derp maps.

The breaking change will remove the configuration flag:

derp_map_path

in favour of a new interface:

derp:
  # List of externally available DERP maps encoded in JSON
  urls:
    - https://controlplane.tailscale.com/derpmap/default

  # Locally available DERP map files encoded in YAML
  paths:
    - derp-example.yaml

  # If enabled, a worker will be set up to periodically
  # refresh the given sources and update the derpmap
  # will be set up.
  auto_update_enabled: true

  # How often should we check for updates?
  update_frequency: 24h

This PR is best reviewed commit by commit.

[juanfont]

Perhaps we should remove this comment

[kradalby]

Do a change suggestion

[juanfont]

why not just one key uri, with file:/// or https:// or whatevs...

[kradalby]

I don't find that particularly intuitive, it's not really a pattern I think most people are used to or use?

Only place I can think of is in browsers

[juanfont]

ok!