policy

OPA does not currently include a built-in function for reversing strings and arrays. While it's possible to write a comprehension that does this, it's a bit cumbersome. We can add two simple built-in functions for reversing strings and arrays:

strings.reverse(str) => str
array.reverse(arr) => arr

There does not seem to be an agreed standard for panic and log messages in Rust, and we already have quite a few inconsistencies within our code base.

I suggest we pick a format and try to stick to it; for instance, this is the closest I could find to a style guide for error messages:
https://docs.rs/serde/1.0.126/serde/de/trait.Error.html#tymethod.custom

The message should not be capital

What are fetch providers?

FetchProviders are the components OPAL uses to fetch data from sources on demand.

Fetch providers are designed to be extendable, and you can easily create more fetch providers to enable OPAL to fetch data from your own unique sources (e.g. a SaaS service, a new DB, your own proprietary solution, ...)

[We have a guide on how to write new fetch providers](https:/