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
- [    ] set properties to `PageService`
- [    ] create Extractor