R: Provide a way to set the default CRAN repositories

[jmcphers]

The R kernel always sets cran.rstudio.com as the default CRAN mirror. This logic lives here:

https://github.com/posit-dev/ark/blob/4e37e46787ccf7c6a6142e705465580e87f3f0c5/crates/ark/src/modules/positron/options.R#L29-L44

It is possible to change the mirror interactively via R's options() command, but difficult to do so in a way that is durable across R versions and environments, especially for server administrators.

We should make it possible for the repositories to be set via an external mechanism, so that a hosting environment can tell the R kernel which CRAN mirror(s) to use by default. This is important in many organizations for performance reasons (e.g. using pre-built binaries) as well as security and compliance (e.g. using only approved/validated R packages).

Implementation notes:

• This should take the form of a command or configuration in the positron-r extension that can be exposed to other extensions
• It would be nice, though not required, if this work resulted in a better configuration experience for package repositories in Positron (e.g. in Settings, a list of mirrors you can see and edit)
• This will require supporting work in Ark to accept the repositories in some format (arg? env var? file?) and apply them to the options() at startup.

[jmcphers]

Triage:

• There's a workaround here to set your options() to use a mirror that has pre-built binaries for your Linux distribution (such as P3M)
• We could make this a lot better w/o all the plumbing by allowing Ark to have logic to use P3M if available.