Try constructor improvements for Delegations/ delegated roles

Currently creating e.g. succinct delegations looks like this

BIT_LENGTH = int(math.log2(NUMBER_OF_BINS))
succinct_roles = SuccinctRoles(
    keyids=[bins_key.keyid],
    threshold=THRESHOLD,
    bit_length=BIT_LENGTH,
    name_prefix=NAME_PREFIX,
)
delegations_keys_info: Dict[str, Key] = {}
delegations_keys_info[bins_key.keyid] = bins_key

targets.signed.delegations = Delegations(
    delegations_keys_info, roles=None, succinct_roles=succinct_roles
)

Looks bad, i'm sure we could get that to something a bit like:

targets.signed.delegations = Delegations(SuccinctRoles.new(NUMBER_OF_BINS, NAME_PREFIX))
targets.add_key(bins_key)

Or alternatively

targets.signed.delegations = Delegations.new_succint(NUMBER_OF_BINS, NAME_PREFIX)
targets.add_key(bins_key)

What makes sense probably depends on what the other delegation types (most importantly paths delegation) will look like...

Comienza leyendo los constructores de Delegations y SuccinctRoles; después, compáralos con los demás tipos de delegación, especialmente con las delegaciones de ruta. El diseño estará completo cuando el proyecto tenga un flujo de construcción más sencillo y acordado para las delegaciones Succinct y el registro de claves que encaje con las demás APIs de delegación.