Adds ability to explicitly include key fields in C* statements so long as:
- the key field is named `ROWKEY`
- it of type `STRING`.

i.e. you can now do:

`CREATE STREAM FOO AS (ROWKEY STRING KEY, ...) ...;`

Conflicting files
ksql-engine/src/test/java/io/confluent/ksql/ddl/commands/CommandFactoriesTest.java
ksql-engine/src/test/java/io/confluent/ksql/ddl/commands/CreateSourceCommandTest.java
ksql-engine/src/test/java/io/confluent/ksql/ddl/commands/CreateStreamCommandTest.java
ksql-engine/src/test/java/io/confluent/ksql/ddl/commands/CreateTableCommandTest.java
ksql-engine/src/test/java/io/confluent/ksql/schema/ksql/inference/DefaultSchemaInjectorTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/KsqlParserTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/SchemaParserTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/SqlFormatterTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/rewrite/StatementRewriterTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/tree/CreateStreamTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/tree/CreateTableTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/tree/TableElementTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/tree/TableElementsTest.java
ksql-rest-app/src/test/java/io/confluent/ksql/rest/server/StandaloneExecutorTest.java
ksql-rest-app/src/test/java/io/confluent/ksql/rest/server/resources/KsqlResourceTest.java

Conflicting files
ksql-engine/src/test/java/io/confluent/ksql/ddl/commands/CommandFactoriesTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/tree/CreateStreamTest.java
ksql-parser/src/test/java/io/confluent/ksql/parser/tree/CreateTableTest.java