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)

[jsdw]

We've removed the substrate-compat feature flag entirely now, and so I'll close this PR. perhaps this is a place we could improve docs or add an example of how to do such a thing though, so i'd be open to thinking of something in that area that would make it clear how to solve this :)