add maven wrapper

[jrivard]

Adds maven wrapper script to the directory-server project. See https://maven.apache.org/wrapper/

From the Maven Wrapper page:

"The Maven Wrapper is an easy way to ensure a user of your Maven build has everything necessary to run your Maven build.

Why might this be necessary? Maven to date has been very stable for users, is available on most systems or is easy to procure: but with many of the recent changes in Maven it will be easier for users to have a fully encapsulated build setup provided by the project. With the Maven Wrapper, this is very easy to do and it's a great idea and initial implementation borrowed from Gradle."

I also updated the wrapper properties to specifically use Maven 3.8.6.

Note: This should not affect any tool or environment using traditional maven to build; it only adds an additional option to run maven via the script and auto-downloaded maven version.