GEODE-7107: Make jetty-server optional in geode-core pom

[jdeppe-pivotal]

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[prettyClouds]

Can you share the why behind this change for some context? Under what circumstances would this jar not be included?

[upthewaterspout]

I share @prettyClouds concerns - how do we know this is really optional? Are we testing that somehow? IMO we should be getting rid of optional dependencies in geode-core in favor of having separate geode modules with the dependencies - like we've done with geode-lucene, geode-redis, etc.

[jdeppe-pivotal]

The dependency on jetty-server also pulls in a transitive dependency on javax-servlet-api. When running in a Spring Boot context, the presence of the servlet api implies that the application is a web application which will attempt to launch an embedded Tomcat server. This may not be what the user intended.