Represents a SAML Subject element within an Assertion.
Ported from the Erlang esaml_subject record.
esaml_subject
@type t() :: %ExSaml.Core.Subject{ confirmation_method: :bearer | :unknown, in_response_to: String.t(), name: String.t(), name_format: String.t() | nil, name_qualifier: String.t() | nil, notonorafter: String.t(), sp_name_qualifier: String.t() | nil }