Add optional backends parameter to benchmarking requests

[lqd]

Opening as draft as I don't know how to test this: how do I simulate locally a request to benchmark a try build @Kobzol? Do I have to manually add rows to the database like in the 18th century?! :p.

This should 🤞 allow choosing which codegen backends we want to use for benchmarks, from the GH benchmarking requests. Hopefully. Maybe. I don't know how to test that it works, I don't know where is the documentation describing how to run these realistic workflows locally.

(Note that I chose to specifically not document the parameter in the help page, until we can test it out a bit in the real world, after this PR is completed)

[lqd]

won't work plot twist

[lqd]

I'm not sure it's necessary, but it seemed safer to not parse for example include= as a Some("") arguments. I'm not sure how that would go in the DB with an empty string, so I also filtered these as being absent from the command.

Probably more ready for review, but I still can't really test this easily. So still draft :3.

[Kobzol]

Looks great, thanks! Left one nit. Since we don't really have a way to tell the users if they choose an invalid backend, I would make the backend parsing a bit more lenient.

[Kobzol]

Looks good, thank you!