feat: add connect templates and simplify JDBC source (MINOR)

[agavra]

Description

Adds some nice defaults and shorthand for creating a JDBC source connector.

Before:

CREATE SOURCE CONNECTOR `jdbc-connector3` 
WITH("connector.class"='io.confluent.connect.jdbc.JdbcSourceConnector',
"tasks.max"='1',
"connection.url"='jdbc:postgresql://localhost:5432/almog.gavra',
"mode"='bulk'
,"topic.prefix"='jdbc3-',
"transforms"='createKey,extractString',
"transforms.createKey.type"= 'org.apache.kafka.connect.transforms.ValueToKey', 
"transforms.createKey.fields"='username',
"transforms.extractString.type"='org.apache.kafka.connect.transforms.ExtractField$Key',
"transforms.extractString.field"='username', 
"key.converter"='org.apache.kafka.connect.storage.StringConverter');

After:

CREATE SOURCE CONNECTOR `jdbc-connector` 
WITH("connector.class"='io.confluent.connect.jdbc.JdbcSourceConnector',
"connection.url"='jdbc:postgresql://localhost:5432/almog.gavra',
"mode"='bulk',
"topic.prefix"='jdbc-', 
"key"='username');

TODO

• Documentation will be part of end-to-end Connector documentation

Testing done

• Unit testing
• End to end local testing

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[vcrfxia]

Thanks @agavra -- LGTM!

[vcrfxia]

Why would this ever be null?

[agavra]

never... it's just that intelliJ complains when I don't check for it because transformValues has an annotation (@NullableDecl)

[vcrfxia]

Why does this implement SupportConnector?