Allow more control over which specs are reused (#42782)

This PR gives users finer control over which specs are reused during concretization.

The value of the `concretizer:reuse` config option now can take an object with the following properties:
- `roots`: true if reusing roots, false if reusing just dependencies
- `exclude`: list of constraints used to select reusable specs 
- `include`: list of constraints used to select reusable specs 
- `from`: allows to select the sources of reused specs

### Examples

#### Reuse only specs compiled with GCC
```yaml
concretizer:
  reuse:
    roots: true
    include:
    - "%gcc"
```

#### `openmpi` must be used from externals, and it must be the only external used
```yaml
concretizer:
  reuse:
    roots: true
    from:
    - type: local
      exclude:
      - "openmpi"
    - type: buildcache
      exclude:
      - "openmpi"
    - type: external
      include:
      - "openmpi"
```

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