Add stdWrap functionality to FilesProcessor

[ducrot]

stdWrap functionality is missing in FilesProcessor. With this change it is possible, for example, to get files from custom fields with slide.

Example (not working without patch)

page {
    10 {
        fields {
            heroimage = TEXT
            heroimage {
                dataProcessing {
                    10 = FriendsOfTYPO3\Headless\DataProcessing\FilesProcessor
                    10 {
                        as = heroimage
                        references.data = levelfield: -1, heroimage, slide
                    }
                }
            }
        }
    }
}

[coveralls]

Pull Request Test Coverage Report for Build 3903908064

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 60.339%

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Totals
Change from base Build 3832666957:	0.0%
Covered Lines:	677
Relevant Lines:	1122

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💛 - Coveralls