align convert experience with other plugins

[kaisecheng]

logstash-filter-mutate and logstash-codec-csv have convert datatype function.
To convert an integer, both require to set

convert => {
  "fieldname" => "integer"
}

However, in logstash-filter-dissect, it uses "int" for integer and convert_datatype instead of convert

convert_datatype => {
  "fieldname" => "int"
}

To improve user experience, this issue request dissect to support "integer" config and rename convert_datatype to convert