Automatically warn/install versions on enter directory when mismatch with current

**Is your feature request related to a problem?**

I am frustrated when I enter a directory containing a tool-version that is not installed and thus not used by asdf, then I don't use the expected tools. I am not warned about the mismatch between required tool (defined in `.tool-versions` and the one in `asdf current`).

Given a directory with a `.tool-versions`
When the user enters that directory or one of its children
Then asdf **warns** and/ask for **installation** of the tools versions defined in `.tool-versions`

**Describe the solution you'd like**

I would like that asdf detects that the current tools are not the one defined in the `.tool-versions` and install them automatically (or ask for install).

**Describe similar asdf features and why they are not sufficient**

asdf correctly **use** the tools but it did not warn/install them automatically

**Describe workarounds you've considered**

A shell plugin could detect that the current tools and the one in .tool-versions (in the hierarchy) do not match.
This plugin can warn the user that something is wrong and ask if it can install the tools.

Nvm (Node Version Manager) has [some instructions to do it](https://github.com/nvm-sh/nvm#calling-nvm-use-automatically-in-a-directory-with-a-nvmrc-file).
This snippet will use or install (depending of the versions of node installed and declared).

**Additional context**

```
$ asdf version
v0.7.8-4a3e3d6
```

Start by comparing the tools declared in `.tool-versions` with the output of `asdf current` while entering a directory or one of its children. Review the proposed shell-plugin workaround and the linked nvm instructions for the directory-entry behavior. Done means asdf detects mismatches and warns or requests installation of the declared versions without silently using different ones.

Written by the indexing model from the issue text.