endpoint configuration for s3 storage

[michalpicpauer]

Subject

Add endpoint configuration option for s3 filesystem.

I am targeting this branch, because these changes respect BC.

Closes #1773 #1706

Changelog

### Added
- Added configuration option filesystem.s3.endpoint

[core23]

Can you please provide a test, so we don't get unexpected exceptions when not defining the key

[michalpicpauer]

Ok, I made some changes. Is it fine to update the existing test with this new key?

[jordisala1991]

Yes please, update the tests and add a new one to test what happens when this new key is configured

[jordisala1991]

Plus it would be nice to update docs with that addition

[michalpicpauer]

Yes please, update the tests and add a new one to test what happens when this new key is configured

The updated test should cover both situations or am I missing something?

[core23]

Please add an other test that does not modify the endpoint and skips this line

[jordisala1991]

Isn't this one already skipping this line? https://github.com/sonata-project/SonataMediaBundle/pull/1774/files#diff-217ffd8f1233ee89cb0db792bd11981dL220-L236

[jordisala1991]

Only half of tests go inside the if

[jordisala1991]

Please squash your commits into 1

[VincentLanglet]

Please review @core23 :)
Does the @jordisala1991 answer is enough for you ?

[core23]

There is still a test missing that is checking for a null endpoint value, but I don't want to block this PR.

[jordisala1991]

Thank you @michalpicpauer