[Filesystem] Add FTP stream wrapper context option to enable overwrite

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Without this change it's not possible to override a file on FTP by calling Filesystem::copy($originFile, $targetFile, true) as PHP/FTP server responds with error like:

fopen(ftp://...@ftp/file.txt): failed to open stream: Remote file already exists and overwrite context option not specified FTP server reports 213 166440 []

TODO: Write an integration tests? How? Use some real FTP server?
@fabpot I've added a comment.