Remove need for casting in rpc endpoint

[BPScott]

When digging into a some errors that were raised in this file I found there was a bunch of as MessageMap[typeof SOMETHING] casting happening because we structured this code in a way that typescript isn't able to infer data shapes on its own.

This PR refactors the contents of this file so that the data variable is typed in a way that when you use the switch statement do different things based upon the value of data[0], the value of data[1] is correctly narrowed to the expected type without the need for casting from any.

Tophat

In vscode inspect the the type of the data / data[1] values in within the switch statement by hovering over them and looking at what autcomplete tells you and see that they get correctly inferred.

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory.
If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.

[lemonmade]

Sorry Ben, lost track of this. I ended up redoing this work with a few less changes to the code structure itself, in order to avoid the complexity of resolving merge conflicts: e701fb0