Centralised accessors for :wallet_passes Application env.

Required keys raise on missing values (apple_pass_type_id, apple_team_id, Apple cert/key/WWDR triple, google_issuer_id, google_service_account_json, repo, pass_data_provider); optional keys (apple_web_service_url, google_callback_url, the _base_url overrides) return nil or fall back to public Apple/Google endpoints. See Getting Started.