Add app config

[sergiimk]

I used a crate called figment that allows layered config loading / merging.

It allows me to specify a config like:

auth:
  providers:
    - kind: dummy
      accounts:
        - accountId: "0"
          accountType: User
          accountName: kamu
          displayName: kamu
          avatarUrl: https://avatars.githubusercontent.com/u/50896974?s=200&v=4

Config can also be overridden with env vars, e.g. KAMU_API_SERVER_FOO=x.