Add batching api for groups

[acrrd]

The PR adds the batching API for groups. Closes: #2326

This is still a draft to get some feedback on the direction of this PR.
The current change adds support for batching groups API.
GRPC, Openapi and tests are still missing.

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[acrrd]

@generall Do you think this is going in the right direction?

[acrrd]

I left this signature as is to avoid changing too much in the tests.

[acrrd]

In this case, execute should always return one element. Would it be preferable to return an error or use unwrap() instead of unwrap_or_default()?

[acrrd]

We can rewrite fn group to use fn group_batch