Fix/primary key only table

[darkmatter18]

Fix

fixes #142

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Description

The Pull request fixes the connector issue, which doesn't support primary key-only tables.

In Cassandra, if there is any table with only primary key columns,

Ex,

CREATE TABLE source.results (
    uuid           text,
    value          int,
	PRIMARY KEY ((uuid, value))
)
WITH cdc = TRUE;

then the connector throws an error because of an error in the SQL select statement formation.

java.util.concurrent.CompletionException: com.datastax.oss.driver.api.core.servererrors.SyntaxError: line 1:7 no viable alternative at input 'FROM' (SELECT [FROM]...)
	at java.util.concurrent.CompletableFuture.reportJoin(CompletableFuture.java:412) ~[?:?]
	at java.util.concurrent.CompletableFuture.join(CompletableFuture.java:2044) ~[?:?]
	at com.datastax.oss.pulsar.source.CassandraSource.batchRead(CassandraSource.java:574) ~[?:?]
	at com.datastax.oss.pulsar.source.CassandraSource.maybeBatchRead(CassandraSource.java:463) ~[?:?]
	at com.datastax.oss.pulsar.source.CassandraSource.read(CassandraSource.java:455) ~[?:?]

Considerations

There is confusion on the issue (#142) about what to push into the data topic when it's only a primary key table, and how to differentiate between a delete query and primary key-only table inserts.

To differentiate between the above-mentioned scenarios, we decided to send all the columns' data into the value field (including primary key columns) for insert and update for a primary key-only table.

The delete query data for all types of tables, including primary key-only tables, will have the value as null for AVRO and {} for JSON, as before.

What's added/changed

• Database query statement is being resolved, which in turn fixes the above-mentioned issue.
• Added the pk columns on the value schema in case of a primary key-only table.

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.

[darkmatter18]

Closing this PR as #199 has the same implementation.