Set up postgresql testcontainers

- add .json to .editorconfig
- add spring-configuration-metadata.json
- update README.md
- add testcontainers dependencies
- add PostgresContainer
- add PostgresInitializer

src/main/kotlin/ltd/hlaeja/test/container/PostgresContainer.kt

@@ -0,0 +1,11 @@
+package ltd.hlaeja.test.container
+
+import org.junit.jupiter.api.extension.ExtendWith
+import org.springframework.test.context.ContextConfiguration
+
+@Suppress("unused")
+@Retention(AnnotationRetention.RUNTIME)
+@Target(AnnotationTarget.CLASS)
+@ExtendWith(PostgresExtension::class)
+@ContextConfiguration(initializers = [PostgresInitializer::class])
+annotation class PostgresContainer