tools: ynl-gen: multi-attr: type gen for string

Add support for multi attr strings (needed for link alt_names).
We record the length individual strings in a len member, to do
the same for multi-attr create a struct ynl_string in ynl.h
and use it as a layer holding both the string and its length.
Since strings may be arbitrary length dynamically allocate each
individual one.

Adjust arg_member and struct member to avoid spacing the double
pointers to get "type **name;" rather than "type * *name;"

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250429154704.2613851-8-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>