Add file (size) validation when files already exist

We could try to enhance the check for file existance by validating the file size. This way we can differentiate fully downloaded files from files where the download was interrupted.

```py
import cdsapi
c = cdsapi.Client()
r = c.retrieve(...) # make a request and start the downloading process
r.content_length # bytes
>>> 65337064
```
`r.content_length` will return the size of the file in bytes. This can be compared to `Path(...).stat().st_size`.

This way we can skip existing files if they match the right `content_length`, and otherwise overwrite them.

_Originally posted by @BSchilperoort in https://github.com/eWaterCycle/era5cli/issues/155#issuecomment-1638227753_