Use GraalVM to create a native image

[timyates]

If we use GraalVM to build a native image of the server, we should be able to greatly reduce startup time, and prevent the occasional timeouts we see with a cold start.

We will use the latest GraalVM v21

• ./gradlew nativeCompile creates a native image when using a GraalVM JDK
• ./gradlew nativeTest runs all the tests against the nativeImage (to shake out errors)
• The native image is manually checked and is runnable
• Github actions to build native image are created