[Security] add an AbstractVoter implementation
feature #11183 [Security] add an AbstractVoter implementation (Inoryy)

This PR was squashed before being merged into the 2.6-dev branch (closes #11183).

Discussion
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[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        | https://github.com/symfony/symfony-docs/pull/4257

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.

Commits
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d3bafc6 [Security] add an AbstractVoter implementation