Spanner: support custom key type encode/decode

[vvakame]

Is your feature request related to a problem? Please describe.
I wanna use custome type with spanner.Key likes #854 / #2614

expected.

type TodoID string

type Todo struct {
  TodoID TodoID
  ...
}

func Update (ctx context.Context, todo *Todo) {
  tx.ReadRow(ctx, "Todos", spanner.Key{todo.TodoID})
  ...
}

We should be write like spanner.Key{string(todo.TodoID)} now. It's annoying & easy to make bug.
compiler can't check this problem.

Describe the solution you'd like
support spanner.Encoder to keyPartValue function and Key#String method.

Describe alternatives you've considered
make utility function likes func toSpannerKey(id TodoID) spanner.Key.

Additional context
I wrote up some code to this issue. I'll submit PR to this repo.