feat: create standalone samples that run against emulator

[olavloite]

Adds a Hello World sample that can run independently without the need for any setup. The sample includes:

• EmulatorRunner: A tool that will automatically run the Spanner emulator in a Docker container and create an instance and a sample database in the emulator. The runner is generic and can be used to create multiple standalone samples.
• HelloWorld sample: The simplest possible sample for how to use the go-sql driver. The sample opens a database connection and selects the string Hello World! and prints this to the console.
• Transactions sample: The existing transactions sample has been refactored to also use the emulator so it can be executed, and example table has been changed to the standard Singers example.
• The existing 'hello' sample has been removed.

[hengfengli]

LGTM.