Resolves configuration for each backend by merging (in increasing precedence):

1. Library defaults declared in this module under @defaults.
2. Application configuration under the :atp_client OTP app.
3. Per-call options passed as a Keyword.t().

Defaults are merged per key, so a partial config :atp_client, :sotptp, … in config.exs only overrides the keys it names; it cannot accidentally clobber :url and leave the library with no endpoint.

Example

In config/config.exs:

config :atp_client, :starexec,
  base_url: "https://starexec.example.org/starexec",
  username: System.get_env("STAREXEC_USER"),
  password: System.get_env("STAREXEC_PASS")

config :atp_client, :isabelle,
  host: "isabelle.example.org",
  port: 9999,
  password: System.get_env("ISABELLE_PASSWORD"),
  # `:local_dir` defaults to a subdirectory of `System.tmp_dir!/0`.
  # Only set it (and `:isabelle_dir`) when the BEAM and the Isabelle
  # server see the shared directory under different paths.
  session: "HOL"

Any setting may be overridden per call. For instance,

AtpClient.Isabelle.query(theory, "Example", session: "Main")

forces the Main session for that single query regardless of what is set in config.exs.