(draft) dte: verify signature over element 'SetDTE' in 'EnvioDTE'

[glarrain]

See:

• https://github.com/LibreDTE/libredte-lib/blob/22c6f8d5d8231ac27977020b8385547a3fd98dbc/examples/008-verificar_enviodte.php#L44-L62
• https://github.com/LibreDTE/libredte-lib/blob/ca8843735df63ea2405042935ac6f7a4a1628d2e/lib/Sii/EnvioDte.php#L422-L438

And for signature verification of each element 'DTE' in 'SetDTE' see:

https://github.com/LibreDTE/libredte-lib/blob/22c6f8d5d8231ac27977020b8385547a3fd98dbc/examples/008-verificar_enviodte.php#L68-L89