Add template for AclProviderInterface

[VincentLanglet]

Hi @ondrejmirtes

If I understand correctly template and I didn't make any mistake, this PR seems better than symfony/security-acl#60.

By the way I have two others questions:

• WDYT about supporting directory in the stubFiles config ? This way the following code would be enough.

stubFiles:
    - stubs/

• WDYT about using the same directory structure than Symfony inside the stub directory ? There will be more and more file, and it can be easier to find them if we follow the namespace.

[VincentLanglet]

I need phpstan/phpstan#3111 first

Opened phpstan/phpstan-src#301

[VincentLanglet]

@ondrejmirtes I removed the findAcls annotation since it requires the next phpstan major.
So this PR is now ready to merge.

[VincentLanglet]

@ondrejmirtes I rebase master ; this PR is ready to merge

[ondrejmirtes]

Hi, I'm sorry, but this PR does too many things at once and it's unlikely to be merged.

1. Adds AclProviderInterface stub - should be in a separate PR.
2. Moves stubs into subdirectories - should be a separate PR.
3. Adds CS to stubs - not interested in this, sorry, as other repositories don't have this and there's little value in that.