CI: add clippy checks

[ordian]

Steps:

• Introduce a list of lints that we don't want to fix straight away similar to https://github.com/paritytech/substrate/blob/master/.cargo/config.toml
• Fix the rest of the clippy warnings: cargo clippy --fix
• Introduce a CI check for clippy similar to Substrate's: https://github.com/paritytech/substrate/blob/6cb4b6799de6f784f4c42eb01a76a8fa67039a67/scripts/ci/gitlab/pipeline/test.yml#L38-L52

[bkchr]

• Introduce a list of lints that we don't want to fix straight away similar to https://github.com/paritytech/substrate/blob/master/.cargo/config.toml

It should be exactly the same config. We should not use different confings in our repos.

[ordian]

• Introduce a list of lints that we don't want to fix straight away similar to https://github.com/paritytech/substrate/blob/master/.cargo/config.toml

It should be exactly the same config. We should not use different confings in our repos.

We could initially allow more lints to fail and fix them gradually.
There are also some lints in Substrate that could be fixed in Polkadot (I haven't actually checked, but in theory), e.g.

"-Aclippy::borrowed-box", # Reasonable to fix this one

We shouldn't wait for substrate to fix it first.

[bkchr]

I just want the same behavior across repos. Especially as a lot of clippy settings are very opinionated stuff or force you to do stuff in way that is more correct in theory, but doesn't change anything in practice.