Convert streaming expressions into packages

Streaming expressions should be converted to packages so that:

1. The SolrJ package is leaner
2. Streaming expressions can have a faster release cycle
3. We can invite more contributions for streaming expressions and also encourage third party streaming expressions.

In this issue, let us achieve the following:

1. Isolate the streaming expressions into a contrib module first so that a jar file can be built out of that. The StreamingHandler can stay in Solr-core.
2. Instead of all the 300+ expressions be added to configset (via solrconfig.xml or Config API), let us build a streaming expressions factory that registers all of the named expressions at once.
3. Eventually, once we're done with the above two steps, we can worry about signing, distribution etc., as per SOLR-14688.