Expose Fineract build's Git revision on /actuator/info HTTP API endpoint

It can be useful to be able to know what exact build of Fineract is running on a given server.

This can be achieved by automatically including Git revision etc. details in a properties file during build time, and then exposing that information on an HTTP API endpoint.

Spring Boot's "Production-ready Features" (formerly known as its Actuator), provides this more or less out of the box; the goal of this issue is to (a) add the required Gradle plugin to the build, (b) do whatever is required so that /actuator/info includes Git details (may be it "just works" out of the box...), and add a few lines of integration test for this. Background:

• https://docs.spring.io/spring-boot/docs/current/reference/html/production-ready-features.html#production-ready-application-info
• https://rieckpil.de/howto-expose-git-information-with-spring-boots-actuator/
• https://medium.com/@d.lopez.j/springboot-maven-jenkins-git-how-to-identify-your-release-automagically-13261ece13f
• https://www.baeldung.com/spring-git-information