Align supported Spanner Interval literal formats

[scotthart]

The Spanner service currently accepts ISO8601 formatted literals as well as a subset of other formats that are supported in Postgres. The google::cloud::spanner::MakeInterval function accepts both ISO8601 formats as well as a larger subet of Postgres formats. It would be nice if MakeInterval and Spanner (via GoogleSQL) were consistent and accepted and denied the same set of Interval literal formats.

[devbww]

This is a non-issue. It is a feature for MakeInterval() to adhere to the robustness principle and be liberal in what it accepts.

The only things to guarantee are that:

• Interval::operator std::string() produces a value acceptable to the service, and
• MakeInterval() accepts values produced by the service (and Interval::operator std::string()).