Explicit parameter generation syntax in templates

- Add "generate" field to specify generator to be used
- Make "value" field the input value for the generator
- Remove parameter "type" field, it's always "string"
I wanted to make the operation idempotent. If we didn't reset the `Generate` field, the result `Value` might have been taken as an input for the next call of `GenerateParameterValues()`. Does it make sense?