[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?
when you submit to master, it is for the next minor version (i.e. 2.6.0).

for bug fixes in patch versions, we don't edit this file