SOLR-13822: Isolated Classloading from packages

[noblepaul]

Description

load plugins from packages
main features:

• A new file for packages definition (/packages.json) in ZK
• Public APIs to edit/read the file
• The APIs are registered at /api/cluster/package
• Classes can be loaded from the package classloader using the <package_name>:<fully.qualified.ClassName> syntax

Checklist

Please review the following and check all that apply:

• I am authorized to contribute this code to the ASF and have removed any code I do not have a license to distribute.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the master branch.
• I have run ant precommit and the appropriate test suite.
• I have added tests for my changes.

[dsmiley]

I have more to review but I looked through 2/3rds of it.

I think the JIRA issue should enumerate the highlights here. For example this issue (as seen from the PR) isn't just the named classloading, it appears to introduce the very notion of a "Package" in the first place (huge deal). It also introduces a syntax for the named classloading for class= using a colon. This is a key thing. Also this adds an API; some endpoints -- a big deal. Needs to be summarized. New stuff is stored in Zookeeper --this should be summarized.