[fsp] Extract ObservedEntry and ObservedEntries to a separate file.
Before they were part of the interface, which should be clean and simple.
Moreover, this CL lets pass the list of changes via ProvidedFileSystemObserver
which will be required soon in the Service class.
TEST=unit_tests, browser_tests: *FileSystemProvider*
BUG=
248427
Committed:
https://crrev.com/b9bac4b512b768e2d17e9199cc1a66997c55aaf3
Cr-Commit-Position: refs/heads/master@{#300071}