Add configurable data dir to config (for pubky-homeserver and pubky-testnet)

[catch-21]

It's not possible to choose the data dir for the homeserver and pubky-testnet always uses an emphemeral tmp dir. It can be useful to point it at a persistent location for some local testing.

[PastaGringo]

Did it on my docker compose and it worked :

  volumes:
      - ./homeserver/config/homeserver.config.toml:/root/.pubky/config.toml
      - ./homeserver/data:/root/.pubky