Feature Request: new RDS util for converting ctx.args to comma separated list of SQL update values

I have an update mutation resolver with the following as its request template:

```
#set ($updates = "")

#if ($ctx.args.name)
#set ($updates = "$updates, name = '$ctx.args.name'")
#end

#if ($ctx.args.duration)
#set ($updates = "$updates, duration = $ctx.args.duration")
#end

{
"version": "2018-05-29",
"statements": [
"UPDATE tracks SET $updates.substring(1) WHERE id = $ctx.args.id AND owner_id = '$ctx.identity.username' RETURNING *"
]
}
```

This works fine, but is a little clunky, and could really get out of hand if I were updating 10+ columns. It would be nice to have a built-in util to convert specific args into a list of column/value pairs for using with SQL's `UPDATE`, e.g.:

```
#set ($updates = $util.rds.argsToValuePairs($ctx.args, ["name", "duration"]))

{
"version": "2018-05-29",
"statements": [
"UPDATE tracks SET $updates WHERE id = $ctx.args.id AND owner_id = '$ctx.identity.username' RETURNING *"
]
}
```

Start with the requested $util.rds.argsToValuePairs helper and the example request template in the issue. Review the existing RDS utility surface and related project conventions before defining the argument-selection and SQL value-pair behavior. Done means the helper supports the shown update use case and its behavior is covered by appropriate project tests.

Written by the indexing model from the issue text.