Do not make pallet-nfts benchmarks signature-dependent

[Moliholy]

This PR:

• Adds extra functionality to pallet-nfts's BenchmarkHelper to provide signers and sign message.
• Abstracts away the explicit link with Sr25519 schema in the benchmarks, allowing parachains with a different one to be able to run them and calculate the weights.
• Adds a default implementation for the empty tuple that leaves the code equivalent.

[bkchr]

polkadot-sdk/substrate/frame/nfts/src/benchmarking.rs

Lines 232 to 236 in ad86209

	where_clause {
	where
	T::OffchainSignature: From<MultiSignature>,
	T::AccountId: From<AccountId32>,
	}

this should also be removed.

[bkchr]

Hmm. This is not a good solution. This way () will not work for any runtime.

[Moliholy]

Current behaviour is that it builds but fails to execute the benchmarks. In my opinion it's already an improvement that it does not build, so that you can take care of the error instead of thinking "benchmarks will be fine because it compiles".

[Moliholy]

this should also be removed.

Nice catch. Done in 54b2397.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-clippy
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6491090