Redesign asset retrieval in the library.

Most assets now return a new class named `Asset`. This allows for the
assets to be consistently saved via a `save` method instead of special
casing for `Attachment`.

`AppInfo` is no longer a namedtuple it is a fully documented dataclass,
as well as having the state attached to it.

Fixes #1997

docs/api.rst

@@ -40,6 +40,9 @@ Client
 .. autoclass:: AutoShardedClient
     :members:
 
+.. autoclass:: AppInfo
+    :members:
+
 Voice
 ------
 
@@ -1918,6 +1921,12 @@ Attachment
 .. autoclass:: Attachment()
     :members:
 
+Asset
+~~~~~
+
+.. autoclass:: Asset()
+    :members:
+
 Message
 ~~~~~~~