Ability to prepare and process more than one EAD field

[geonnave]

Currently one either 0 or 1 EAD fields can be added to a message. This is not complete according to the EDHOC RFC, which specifies 0 or N EAD fields.

From the API perspective, here is one possible way to implement it (see this review comment):

let (state, msg1) = start.prepare_message1(x, g_x, c_i)
    .add_ead1(id, Some(bytes))?
    .add_ead1(more_id, None)?
    .finish(&mut crypto);