ci: Add dependabot for dependency management.

[afifurrohman-id]

It would be nice if dependabot can open PR for update dependencies for axum repository, such as using cargo or GitHub Actions too.

[jplatte]

What's the motivation for this?

[wangziling]

The dependabot can automatically check the dependencies' version daily or other durations.

E.g. https://github.com/salvo-rs/salvo/blob/main/.github/dependabot.yml
salvo-rs/salvo#751 (comment)

It can notice the project owner that some dependecies are upgraded for fixing/enhancing something.

Maybe this discussion initiator wanna this project can grasp the changing events of its related crates.😉

[jplatte]

Well, I think we're already generally very well aware of when dependencies can use upgrades. Actually doing the upgrades can sometimes take time, but not because we don't know that there is something to update :)