Do not make pallet-nfts benchmarks signature-dependent (#4756)

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.