bump overlay: support overwriting the input OpenAPI file

I have several large overlay files that I'm applying one by one and I'd like to have an option to force update the input OpenAPI file in-place. This will help simplify my automation scripts by eliminating the need to create and then delete multiple temporary files with intermediate results.

`--out=INPUTFILE` works but triggers an interactive confirmation prompt which I think isn't compatible with CI/CD.

`> INPUTFILE` does not work because it blanks the file before `bump` can read it, resulting in an error.

`| sponge INPUTFILE` is a possible workaround for macOS/*nix users, but we also have Windows users who can't use `moreutils` so it's not an option.

----

**UPD:** I found a way to silently overwrite the input spec on macOS/*nix:

```sh
echo y | bump overlay myspec.yaml overlay.yaml --out=myspec.yaml
```
but it would still be great to have a built-in option for cross-platform use.

Start at the `bump overlay` command and trace how `--out=INPUTFILE` handles confirmation and input/output paths. Define the in-place overwrite option's behavior for noninteractive CI/CD use, including the cross-platform case, and verify that applying an overlay updates the input file without prompting or truncating it first.

Written by the indexing model from the issue text.