[Security] add an AbstractVoter implementation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | TODO

The idea is to reduce boilerplate required to create custom Voter, doing most of the work for the developer and guiding him on the path by providing simple requirements via abstract methods that will be called by the AbstractVoter.

P.S. This is meant to be a [DX Initiative](https://github.com/symfony/symfony/issues?labels=DX&state=open) improvement.
> so there's no difference whether you "cache" a value or pull it from a getter every time

Well, the difference is pretty minimal, but it's still there (and will only get worse with the "bad" getter impelemtation). So the only advantage of the repeated getter call, so far as I see, is -1 loc.