Support search by PK without the need to specify column names

In DAB it is possible, via REST, to get a specific item by specifying the PK in the URL path, using a `/column1/value1/.../columnN/valueN` pattern.

We should evaluate the ability to have an option in the configuration file, say `positional-pk` for example:

```
"rest": {
"path": "/api",
"positional-pk": true
}
```

that would allow to search by PK using only the values and omitting columns names, for example, from this

```
/api/todo/id/1
```

to

```
/api/todo/1
```

which is very common in REST calls