fx.Annotate: Variadic options should be optional

[josephinedotlee]

fx.Annotate allows passing a function that takes a variadic number of
arguments, and feeding them from a value group:

func NewFoo(...FooOption) *Foo

fx.Annotate(
  NewFoo,
  fx.ParamTags(`group:"option"`),
)

However, there's a bug: if you do not use the value group,
the parameter is a required []FooOption.

For example,

func NewFoo(...FooOption) *Foo

fx.Annotate(
  NewFoo,
  fx.As(new(Fooer)),
)

In this, the FooOptions should be optional as it is in fx outside of Annotate.

Whenever no tags are specified for a variadic parameter, it should be optional by default.