Add dynamic extrinsic signature submission

[haerdib]

Follow up issue of #344

Currently, the user can only submit extrinsic statically. E.g. the signature is known at compile time. While we want this type-safety, offering a dynamic version of signing the extrinsic could have its benefit.

See https://github.com/scs/substrate-api-client/issues/344#issuecomment-1960946948 for more information.