Allow clippy::unit_arg lint in generated code

[jebrosen]

Fixes #1495.

This PR can be verified with cargo clippy -p request_local_state -- --no-deps, which shows two unit_arg warnings in the example before this PR and no warnings after.

This is tricky to test in CI in the current state, since we have some failing clippy lints in rocket, rocket_http, etc (some of which are "errors" in the default configuration).

[SergioBenitez]

Can we simply allow all clippy lints for generated code? We could gate this in such a way that we can selectively enable clippy linting if required.

[jebrosen]

I believe we could insert #[allow(clippy::all)] in a few places - certainly for the autogenerated #[route] and #[catch] code which is the most likely to run into lints. I'm not sure if lint attributes are allowed in every single place we generate code (in particular I'm worried about uri!), but let's see how far we can get!

We could gate this in such a way that we can selectively enable clippy linting if required.

Would this part be needed right away? We could do another environment variable e.g. ROCKET_CODEGEN_UNMASK_CLIPPY=1

[SergioBenitez]

Would this part be needed right away? We could do another environment variable e.g. ROCKET_CODEGEN_UNMASK_CLIPPY=1

I was intending as a cfg, to the effect of the existing rocket_unsafe_secret_key, so that we can enable it in scripts when needed, perhaps even as a alllow-fail CI task.

[SergioBenitez]

This appears to no longer be required: #1495