Support uploading artifacts to an existing draft release for better integration with Immutable Releases

GitHub supports [`Immutable Releases`](https://docs.github.com/code-security/concepts/supply-chain-security/immutable-releases), which prevents changes to artifacts after a release has been made public.

Some projects manage their release lifecycle by splitting it into two tools. One tool manages release creation (e.g., release-please), and the other handles building and publishing (e.g., GoReleaser, **cli-gh-extension-compile**).

In such configurations, when the immutable release feature is enabled, it is common for the release management tool to create a tagged draft release, and for the build tool to upload artifacts to that draft release and perform the publication; however, this action does not support that workflow.

I propose a new `use_existing_draft` input (default: `false`) to address this.
When set to `true`, the action looks up the release for the tag, verifies it is in draft state (and fails clearly if not), uploads artifacts, then publishes it with `gh release edit --draft=false`.