Validated configuration for MPP.Plug.
Built once at init time from plug opts. Holds shared endpoint settings
and a list of MethodEntry structs — one per accepted payment method.
Summary
Types
@type t() :: %MPP.Plug.Config{ expires_in: pos_integer() | nil, method_entries: [MPP.Plug.MethodEntry.t()], opaque: String.t() | nil, realm: String.t(), secret_key: String.t() }