[Bug]: Multiple coverage reports overwriting comments

Just raising a nit bug (?) I found.

When using multiple coverage actions, based on https://github.com/insightsengineering/coverage-action/issues/35, I'm passing a different `storage-subdirectory` so that the saved XML reports on `diff-storage` branch don't overwrite each other.

However, another hidden requirement for multiple actions is a different `path`, since it's used as the sticky comment's unique key (`header`):

https://github.com/insightsengineering/coverage-action/blob/50449af77a423ca017bf5496a6711db404d31811/action.yml#L356-L362

It would be nice to either:

1. Create a separate input to handle the stick comments unique keys; or
2. Document on the `path` description that it's used as the sticky comment unique key; or
3. Use `storage-subdirectory` as the unique key instead? Though it might fall into a similar problem we have with the current `path` hidden behavior, and may limit user shenanigans in case they want to have multiple actions overwriting each other's comments.

Sorry for the flood lately, no hurry here, as for now I can just pass different paths to the actions (which might be the intended way from the start).

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