R: Provide a way to register R RPCs from the R extension

Today, if you want to call R from the R extension, it is easier than it used to be, but still a real chore. You must:

• write the implementation in one of the R files in the amalthea project (e.g.: https://github.com/posit-dev/amalthea/pull/194)
• get it reviewed and merged into main
• bump the amalthea version
• wait for successful build
• coordinate a merge into positron

Not only is this a lot of paperwork, it also creates unnecessary coupling between Positron and ark: because the RPC invocation and implementation live in different repositories, it is much easier for them to be out of sync (e.g. parameters or return types mismatched).

I propose that we:

• add a registerRpc method to ark that can be called in https://github.com/posit-dev/positron/blob/main/extensions/positron-r/resources/scripts/startup.R.
• define RPCs in the R extension itself rather than in ark's R modules

In this way, we can keep some RPC invocations and implementations in the same repository, and reduce some coupling and paperwork when working on code with feet in both worlds.

Start with the proposed call site in positron-r/resources/scripts/startup.R and inspect how ark currently defines and invokes R RPCs. Determine the registration API and repository boundaries needed to keep definitions and implementations in the R extension; done means an R extension can register and invoke its own RPCs without changes to ark's R modules or a coordinated amalthea release.