Guava was removed from Elasticsearch many years ago, but remnants of it
remain due to transitive dependencies. When a dependency pulls guava
into the compile classpath, devs can inadvertently begin using methods
from guava without realizing it. This commit moves guava to a runtime
dependency in the modules that it is needed.

Note that one special case is the html sanitizer in watcher. The third
party dep uses guava in the PolicyFactory class signature. However, only
calling a method on the PolicyFactory actually causes the class to be
loaded, a reference alone does not trigger compilation to look at the
class implementation. There we utilize a MethodHandle for invoking the
relevant method at runtime, where guava will continue to exist.