Add validation constraints to core.extension

Problem/Motivation

Over at #1170362: Install profile is disabled for lots of different reasons and core doesn't allow for that, the schema for core.extension:profile is being refined.

Once that lands, we'll be able to make it validatable too, and make the entire config object validatable!

Conversation that triggered this issue to be created:

Wim

IMHO this is missing the following validation constraints, because right now **literally any string is allowed**:
```suggestion:-0+0
      label: 'Install profile'
      constraints:
        NotBlank: []
        ExtensionName: []
        ExtensionExists: profile
```

1. `NotBlank: []` because the empty string is never a valid install profile
2. `ExtensionName: []` because that validates the _shape_ of allowed strings
3. `ExtensionExists: profile` because that validates that (the already valid shape) indeed exists

Note that these 3 constraints are _exactly_ what we do for validating installed modules and themes in `config_dependencies_base`.

The only thing that's missing for that to work is a tiny addition to `ExtensionExistsConstraintValidator::validate()` using `\Drupal\Core\Extension\ProfileExtensionList`?

P.S.: _maybe_ `ExtensionExists` is impossible due to a race condition, but the other two validation constraints should be possible.

Alex

I think we should explore this in a follow-up. This is not really in-scope here.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet