Allow ksql-datagen's msgRate to have slower cadences.

[pavel-kalmykov]

Is your feature request related to a problem? Please describe.
When running ksql-datagen with the msgRate parameter using a decimal number, I get the following error:

Invalid integer value '0.2'; must be a valid base 10 integer
usage: DataGen 
[help] 
[bootstrap-server=<kafka bootstrap server(s)> (defaults to localhost:9092)] 
[quickstart=<quickstart preset> (case-insensitive; one of 'orders', 'users', or 'pageviews')] 
schema=<avro schema file> 
[schemaRegistryUrl=<url for Confluent Schema Registry> (defaults to http://localhost:8081)] 
key-format=<message key format> (case-insensitive; one of 'avro', 'json', 'kafka' or 'delimited') 
value-format=<message value format> (case-insensitive; one of 'avro', 'json' or 'delimited') 
valueDelimiter=<message value delimiter> (case-insensitive; delimiter to use when using delimited value format. Must be a single character or special values 'SPACE' or 'TAB'. Defaults to ',')
topic=<kafka topic name> 
key=<name of key column> 
timestamp=<name of timestamp column> 
[iterations=<number of rows> (if no value is specified, datagen will produce indefinitely)] 
[propertiesFile=<file specifying Kafka client properties>] 
[nThreads=<number of producer threads to start>] 
[msgRate=<rate to produce in msgs/second>] 
[printRows=<true|false>]

This validation, of course, makes it obvious that the parameter's type must be an integer. Because of this validation, the message cadence cannot go below 1 msg/sec.

Describe the solution you'd like
By allowing decimal numbers, we could have a slower cadence. For instance, if I wanted to generate one message every 10 seconds, I would set msgRate to 0.1.

Describe alternatives you've considered
Another possibility would be to allow specifying a time unit (seconds, milliseconds, s, ms...). Both proposals are not mutually exclusive.

Additional context
I am aware that the maxInterval parameter was deprecated in favour of this one; however, the former had milliseconds as a unit, making it feasible to configure a 1msg/10secs cadence, and forcing the usage of msgRate (with seconds as its unit) comes across as a regression. Additionally, having this tool parameter's types for the help command and the site's documentation would be more than helpful.