dotnetup should support `--dry-run` for all commands that may modify the system

### Prerequisites

- [x] I have read the [dotnetup documentation](https://github.com/dotnet/sdk/tree/release/dnup/documentation/general/dotnetup).
- [x] I have searched for [existing dotnetup issues](https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3Adotnetup).
- [x] I have searched for [existing dotnetup discussions](https://github.com/dotnet/sdk/discussions/categories/dotnetup).

### Issue type

Bug report

### Description

It's common for tools of all kinds to support a 'what if'/'dry run' mode where the tool will do investigation/prep work for the command the user entered and then give a report of what _would_ happen if the user ran the command.

This can be used for things like update checks, or for building tooling on top of dotnetup.

At minimum,
* sdk install
* sdk update
* sdk uninstall
* runtime install
* runtime update
* runtime uninstall

should support this mode.

In this mode we'd do all of the non-modifying parts of these operations:
* manifest checking
* version resolution (channel -> latest in that channel)
* comparisons to locally installed components
* lists of locally-installed components to be removed/modified
and emit those, but not actually download/remove/install the components.\

(This was requested by @Redth)

### dotnetup version

latest

Start at the dotnetup command entry points for SDK and runtime install, update, and uninstall, then trace manifest checks, version resolution, local comparisons, and system-modifying actions. Done means every listed command reports the planned operations without downloading, installing, removing, or otherwise modifying the system.

Written by the indexing model from the issue text.