Adds the ability to return a Explanation for a streaming expression. The explanation will describe features of an expression including its children. This information can be used by a client to introspect how their expression is interpreted on the server.
It will be used
SOLR-8918 to add Streaming to the admin page.
This does change the interface Expressible to add a new required function which functions similarly to the toExpression(StreamFactory) function. It will return an Explanation object describing that expression.