Add dependabot

[oriontvv]

This PR adds dependabot config

[Darksonn]

What is the motivation for this?

[oriontvv]

Hello. The main motivation is to remove duplicated transitive dependency crates with multiple versions for dependent crates(keep size and build time) and check from time to time in ci if any update of our dependencies could break the pipeline.

[Darksonn]

We don't check in our Cargo.lock, so there's no need for automatic updates of Cargo.lock. As for the Cargo.toml, those should not be automatically updated. We intentionally specify the minimum versions that we are compatible with.

[oriontvv]

Ok, thanks for clarification.