Explicit parameter generation syntax in templates

- Add "generate" field to specify generator to be used
- Add "from" field to specify the input value for generator
- Remove "type" field, as it's always "string" anyway

In short, it makes parameter syntax:

```
{
    "name": "Param",
    "generate": "expression",
    "from": "[a-z]{10}",
    /* "value":  <-- To be generated */
}
```

instead of

```
{
    "name": "Param",
    "expression": "[a-z]{10}"
}
```
addressed.