Configuration loading and typed accessors.

Reads `resources/conf/<env>/config.edn` through Aero and exposes the
settings libraries need. Assembling an Integrant system from those settings
is the application's job — see `wagoe.system-config` in the monorepo, or the
`config.clj` that `wagoe new` generates.

This lives in a library because four published libraries used to reach for
it with `requiring-resolve`, which worked in the monorepo and in generated
projects and nowhere else — and `wagoe.user`'s CLI resolved a `db-spec` that
generated projects never had (BOU-306).

Usage:
  (def config (load-config))
  (db-spec config)

The `#secret` Aero reader and the sources it resolves through.

  #secret "DB_PASSWORD"                         ; the default source, :env unless configured
  #secret [:file "db-password"]                 ; a named source
  #secret {:name "DB_PASSWORD" :default "dev"} ; a default, for dev

A source is anything implementing `ISecretSource`. `:env` and `:file` are
built in; declare more under `:secret-sources` in the config file, or pass
them to `wagoe.config/load-config`. A secret no source has fails the read,
naming the secret and never a value.