Add update checker (#3855)

Co-authored-by: MD <1917406+mdcfe@users.noreply.github.com>

Adds an update checker to Essentials that will check for the latest version on startup, on player join (permission is `essentials.updatecheck`), or manually with `/ess version`.

On startup, the update checker will fetch build information from a resource generated at compile time and determine if the build is a dev or stable build. If it's a stable build, the update checker will only check for a new release; while a dev build will check for any new commits.

There are 6 different types of messages the update checker will return;
* Identical: The current build is the latest stable release or latest dev build. This message is only shown in the `/ess version` command.
* Behind: If the current build is stable, it's an entire stable build behind, otherwise it's one or more dev builds behind.
* Diverged: The current build was made from a branch other than `2.x` and is also one or more dev builds behind the latest commit on `2.x`.
* Diverged Latest: The current build was made from a branch other than `2.x` but is based on the latest commit from `2.x`.
* Unknown: The current build either has invalid build information or was customly built. This message is show everywhere but on player join.
* Error: There was an error while fetching the latest version information.

Update checks can be disabled using the `update-check` option in `config.yml`.

Essentials/src/main/java/com/earth2me/essentials/ISettings.java

@@ -391,6 +391,8 @@ public interface ISettings extends IConf {
 
     boolean isRespawnAtBed();
 
+    boolean isUpdateCheckEnabled();
+
     enum KeepInvPolicy {
         KEEP,
         DELETE,