IBX-1439: Refactored SiteAccess-aware Flysystem Adapter to use proxying

[alongosz]

Question	Answer
JIRA issue	IBX-1439
Target PR	#171
Type	feature
Target Ibexa version	v4.4
BC breaks	yes

This PR replaces 1:1 copy of Flysystem's Local Adapter with the solution suggested by Flysystem's Maintainer - an Adapter which is decorating Local adapter instances which use / as filesystem root, allowing to proxy all method calls, prefixing the path with our logic beforehand. See #171 for more details about the suggested solution.

Note that given currently SiteAccess dynamic setting io.root_dir is always an absolute path, the inner adapter is going to work only if its prefix is indeed /.

TODO:

• Check if 1st party IbexaTestKernel-dependent tests are still working

Checklist:

• Provided PR description.
• Tested the solution manually.
• Provided automated test coverage.
• Checked that target branch is set correctly
• Ran PHP CS Fixer for new PHP code (use $ composer fix-cs).
• Asked for a review

[alongosz]

Merging this one into #171