Introduce dependabot

[tisonkun]

Dependabot supports automatically updating Cargo dependencies.

databend and tikv/yatp already makes use of it and I think it's a net win to introduce such bot for Engula so that we don't have to update dependencies manually.

A prerequisite is that @huachaohuang whether you think we should keep all dependencies up-to-date? If so, please mark this issue as good-first-issue so that contributors can deal with the details. If not, close it with explanations.

[PsiACE]

databendlabs/databend#3785

emmm, Dependabot is good, but not enough

[huachaohuang]

I don't think this is necessary for now, considering that the current codebase is still very unstable. It seems a nice thing to have in the future, though.

[huachaohuang]

Close this for now until we have a more stable codebase.