Now that Envoy is using c++17, std::string_view is available.

To avoid a huge change to replace all uses in the code, absl::string_view will become an alias for std::string_view if we #define ABSL_USES_STD_STRING_VIEW (see https://github.com/abseil/abseil-cpp/blob/d39fe6cd6f5eb72dc741f17d3a143a6a5a56538a/absl/strings/string_view.h#L45).

New code should use std::string_view. Code, as it is modified, should be slowly converted to std::string_view. clang_tidy or check_format should stop allowing new uses of absl::string_view.

Same thing for absl::optional --> std::optional, using ABSL_USES_STD_OPTIONAL