Add Java 11 & MariaDB Definition

[elaine-jackson]

This pull request adds a Java 11 & MariaDB Definition. It is adapted from https://github.com/microsoft/vscode-dev-containers/pull/292 (which was made for PHP) and works similarly.

This was tested with a private Java 11 Spring Boot Application with no modifications necessary. As long as you have the MariaDB Driver and pull MYSQL_HOST as the environment variable for your hostname this should work out of the box.

I appreciate any feedback maintainers are able to give me.

[ghost]

All CLA requirements met.

[elaine-jackson]

I setup port options for developers who want to change the port (e.g. if they run MySQL locally). Should we expose MySQL to the system by default? I think it's useful for developers who have tools such as JetBrains Datagrip installed to have an easy way to connect to the Dockerized MySQL installation but is this a bad practice (e.g. should we keep it commented out by default)

[bamurtaugh]

Thanks for opening this PR!

As a heads up, our team has been actively focused on an updated plan for community contributions and this repo moving forward, which we've now outlined in this issue: https://github.com/microsoft/vscode-dev-containers/issues/1589. This includes moving to a couple new repos for images (https://github.com/devcontainers/images) and Features (https://github.com/devcontainers/features).

We anticipate to have a similar repo and distribution process for templates/definitions. We'll keep everyone updated (likely via another issue in this repo or comment on https://github.com/microsoft/vscode-dev-containers/issues/1589) when our new templates repo is available and the process is defined.

Please let me know if you have any questions, thank you!