Additional Mango-based update handler / VDU functionality

This ticket is intended to gather ideas for what other sorts of update handler / VDU functionality we could add to Mango to provide a declarative replacement for JS-based update handlers and VDUs. This is intended as a "blue-sky" ticket to gather ideas that can be implemented simply in a declarative fashion.

From #1534, I wrote:

> There are other proposals to provide similar/useful functionality (#1532, #1533, #1498) if the operator decides to run without a JS engine (#1513), plus I suspect there are thoughts about doing Mango-based update functionality to filter fields, as well as apply database schema enforcement (type and format checking) in a declarative fashion for a NoJS mode. Could your embedded RasPi update function be written declaratively? I assume you're adding info such as incoming IP address, central timestamp, etc. which I could see being done automatically quite easily. This is a use case we can accelerate without the use of JS readily; we just need to agree on an API and how to write it down in a JSON blob.

Extracting:

* Add datetime stamp (#1533)
* Add IP address from incoming host, or other fields from the `req` object
* Add additional content based on fields in the `req` object
* Perhaps a lookup table that maps a `req` field value to a key:value pair (or JSON Patch) to insert?
* Filter out (blacklist) or only allow (whitelist) certain fields (lists) - this is partial JSON schema support
* Type-checking (field `type` is string, field `date` is ISO datetime|UNIX epoch, _etc._)
* Field validation (format-checking, i.e. string must match a regex like `"$###.##"` or an integer must be within a certain range)
* Consider using output of another Mango index/query for validating a field? For instance, a Mango query of documents that are of type `source` and extracting the field `ip_address`, then only allowing documents to come through that have a matching `req.ip_address` (for example).
* Same as above, but to implement the lookup table. Example: look up source's IP address, auto-add name of sensor which is in the first document that matches in the Mango index