This allows users to pass a collection of string-like objects in an
`initializer_list` without having to convert everything to the same
type first.

`initializer_list` has the requirement that everything is copied in to
the list. For strings hitting the `string_view` overload avoids
unnecessary copies.

This may be a breaking change if `absl::StrJoin` has an explicit
template parameter, for example `absl::StrJoin<std::string>({foo,
"bar"});`. In this case, remove the explicit template parameter.

PiperOrigin-RevId: 633295575
Change-Id: Ie5f0860f409f639a27a58949842ec961e0b3bdeb