[WIP] add arbitrary property to seo metadata

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #131 
| License       | MIT
| Doc PR        | add to https://github.com/symfony-cmf/symfony-cmf-docs/pull/442 when ready

* [x] add arbitary property to seo metadata
* [x] set properties to `PageService`
* [x] create Extractor
* [x] admin support in the extension
* [x] add chanLog Entry
* [x] add type to the property value object
* [x] refactor class name
* [x] add type selection to admin form
* [x] train `SeoPresentation` to put the properties int to `PageService` depending on that type
* [x] other CS fixes