[Issue 3884] [pulsar-io] Add a Pulsar IO connector for Solr sink

[murong00]

Motivation

Fixes #3884

Provides a builtin solr sink Connector, in order to persist pulsar messages to a solr collection in some application cases.

Modifications

Add a new sub-module in the pulsar-io module.

Verifying this change

This change can be verified as follows:

• deploy the solr sink connector with configuration file containing the following fields:

configs:
    solrUrl: "localhost:2181"
    solrMode: "SolrCloud"
    solrCollection: "techproducts"
    solrCommitWithinMs: "10"
    username: ""
    password: ""

• deploy an SolrCloud cluster and create the above collection
• send messages in the topic with specified schema declared when deploying the connector
• query solr to check if the messages have been written in the specified collection

[jiazhai]

nit: please remove empty lines

[murong00]

Done, thanks.

[jiazhai]

run java8 tests