Allow to copy files and symlinks while overwriting the destination (#3508)

Adds capability to NIOFS to copy regular files and symlink, allowing to
overwrite the destination.

### Motivation:

Per #3403 and
#3470 we want to add
`replaceExisting: bool` to `FileSystem.copyItem`.

### Modifications:

1. Adds `replaceExisting: bool` parameter to
`FileSystemProtocol.copyItem`.
2. Adds `replaceExisting: bool` parameter to `FileSystem.copyItem` and
implementation for regular files and symbolic links.
3. Adds tests.

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Co-authored-by: George Barnett <gbarnett@apple.com>

Author: stepan-ulyanin