asdf 0.16.0+ `asdf shell` alternative

### Is your feature request related to a problem? Please describe

Before 0.16.0, it was quite handy to be able to set a version for the current shell session. e.g. for no downtime ruby deployments, I could install the new ruby version, use `asdf shell` to switch to it without the current shell, and install the required rubygems via bundler before switching our app over to use it.

### Describe the proposed solution

Reimplement `asdf shell` (or `asdf set --shell`) so that asdf versions can be overwritten for the current shell environment

### Describe similar `asdf` features and why they are not sufficient

With `asdf shell` gone, the only solution is to use `asdf set`, which overwrites tool-versions, which means when the application next boots up or scales up automatically, it uses the changed ruby version which might not be ready yet.

### Describe other workarounds you've considered

I guess a work around would be to make a new directory, create a tool-versions there, a copy over the Gemfile and Gemfile.lock, but this is cumbersome compared to a quick `asdf shell ruby 3.4.1` (or using the new set, `asdf set --shell ruby 3.4.1`)

Start by tracing the existing `asdf set` and shell-command handling, focusing on how versions are resolved and exported to the current shell. Implement the requested `asdf shell` or `asdf set --shell` behavior without changing the project’s tool-versions, then verify that the selected version applies only to the current shell session.

Written by the indexing model from the issue text.