Handle Pgvector itself in new/1

[nallwhy]

If you try to take Pgvector from A and update it in B, you will get the following error.

**(FunctionClauseError)nofunctionclausematchinginPgvector.new/1ThefollowingargumentsweregiventoPgvector.new/1:
# 1
Pgvector.new([...])Attemptedfunctionclauses(showing2outof2):
defnew(list)whenis_list(list)defnew(tensor)whenis_struct(tensor,Nx.Tensor)(pgvector0.2.0) lib/pgvector.ex:11: Pgvector.new/1(pgvector0.2.0) lib/pgvector/ecto/vector.ex:8: Pgvector.Ecto.Vector.cast/1(ecto3.10.3) lib/ecto/changeset.ex:814: Ecto.Changeset.cast_field/9(ecto3.10.3) lib/ecto/changeset.ex:766: Ecto.Changeset.process_param/9(elixir1.15.4) lib/enum.ex:2510: Enum."-reduce/3-lists^foldl/2-0-"/3(ecto3.10.3) lib/ecto/changeset.ex:744: Ecto.Changeset.cast/6

Since Elixir primitives like URI can receive themselves as a parameter of new function, Pgvector does the same thing.

[ankane]

Awesome, thanks @nallwhy! (and thanks @josevalim for the review)