[feature]: config flag to disable adding the default universes to our federation

[jharveyb]

Is your feature request related to a problem? Please describe.

If I start tapd with a default config, a default universe run by LL will be added to my universe federation. If I later want to remove the universe from my federation, I could use a command like tapcli u f d. If I restart my node, the default universe is re-added to my config:

https://github.com/lightninglabs/taproot-assets/blob/d70bccd2714a3f808e070a080c510cf396a11284/tapcfg/server.go#L197

Discovered while manually testing #1014

Describe the solution you'd like

A new config flag to gate this behavior / turn off these defaults, so tapd is usable with an empty federation.

Describe alternatives you've considered

Additional context