Handle RestRequest arguments in a (more) type safe way

Nobody has claimed this yet.

`RestRequest` has an argument map that we use for storing request headers, query params, cookies, and context objects that are set inside of our business logic. Currently, reading these arguments requires an unsafe cast from `Object` to whatever type the argument is supposed to be.

If we used something similar to Netty's `AttributeMap` system, the key could include the type of the value and the cast in the business logic could be avoided (`AttributeMap` still does a cast internally, but this is a little bit cleaner since the casts aren't spread out across the codebase).

### Inspiration
- [AttributeMap javadoc](https://netty.io/4.1/api/io/netty/util/AttributeMap.html)
- [DefaultAttributeMap](https://github.com/netty/netty/blob/4.1/common/src/main/java/io/netty/util/DefaultAttributeMap.java)

Start by reading the RestRequest argument map and the linked Netty AttributeMap and DefaultAttributeMap references. Define completion as request arguments exposing typed keys so callers no longer need unsafe Object casts, while preserving support for headers, query parameters, cookies, and context objects.

Written by the indexing model from the issue text.