Support conditional use of Abseil's string_view and optional, for Envoy.

[dgoffredo]

envoyproxy/envoy#23958 revealed that Envoy's linter doesn't like std::string_view or std::optional, preferring the Abseil absl::* versions instead.

For std::string_view, there is a NOLINT exception, but for std::optional there is not.

This pull request adds a level of indirection for each of string view and optional, namely src/datadog/{string_view,optional}.h, which define the type and type template datadog::tracing::StringView and datadog::tracing::Optional, respectively.

The default behavior, when the DD_USE_ABSEIL_FOR_ENVOY preprocessor macro is not defined, is for the Datadog-specific types to be aliases for the std types.

In Bazel builds, such as Envoy's, DD_USE_ABSEIL_FOR_ENVOY is defined. This make the Datadog-specific types aliases for the absl types.