feat(utils): add MultiSigner helper

[FlorianFranzen]

This add the MultiSIgner to the types of supported multi types.

[jsdw]

Thanks for the PR! Do you have any example of where this would be useful to you? I'm generally a bit hesitant to add things that rely on substrate-compat since it would be ideal to remove that feature completely at some point (especially since Subxt rarely keeps in line with sp_* releases, and chains will also be using various different versions)