Use canBeEnabled in Configuration for enabling

This allows users to enable the config in 3 ways:

````yaml
foo_bundle:
  persistence:
    phpcr: ~

foo_bundle:
  persistence:
    phpcr: true

foo_bundle:
  persistence:
    phpcr:
      enabled: true
      ...
````

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -