Group RustCrypto crate updates.

[reitermarkus]

Changes

Add group for RustCrypto crates.

Context

The RustCrypto org hosts multiple repos containing multiple crates which depend on each other, so they should be updated together.

Documentation (please check one with an [x])

• [ ] I have updated the documentation, or
• [x] No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• [ ] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [ ] Both unit tests + ran on a real repository

Tried on a real repository, but this does not seem to work since I guess the source URL for crates seems to be https://crates.io/crates/<crate> instead of the GitHub repository.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[reitermarkus]

I guess this will only work once https://github.com/renovatebot/renovate/issues/3486 is implemented.

[rarkins]

Closing due to staleness