Add support for additional remote code execution environments, e.g. Riza

I'd like to play with this in some projects/demos for Riza (https://riza.io), which can remotely run Python/JS in isolation via an API. I'm happy to make an attempt at adding support myself, though most testing will be manual.

> 1. What is the *motivation* behind this feature? Is it related to a problem or frustration with the library? Is it
a feature related to something you need for a project? Is it something you worked on and think it could benefit
the community?
>
> Whatever it is, we'd love to hear about it!

I work on Riza so this is specifically useful to me and my team. But having multiple remote execution options would be generally helpful.

> 2. Describe your requested feature in as much detail as possible. The more you can tell us about it, the better
we'll be able to help you.

The feature is just to add a class named `RizaExecutor` which the `CodeAgent` could use for remote code execution. See proposed code snippets below.

> 3. Provide a *code snippet* that demonstrates the feature's usage.
```py
agent = CodeAgent(..., use_riza_executor=True)
# Or perhaps to make this more flexible
agent = CodeAgent(..., executor=RizaExecutor())
```

> 4. If the feature is related to a paper, please include a link.
N/A

Start at the CodeAgent entry point and inspect how executor selection is currently exposed, comparing existing execution options before defining the RizaExecutor boundary. Confirm the intended configuration and manually exercise remote Python and JavaScript execution through the Riza API, with the feature usable from one of the proposed agent setups.

Written by the indexing model from the issue text.