Use RuntimeMetadataPrefixed instead of RuntimeMetadata

[chevdor]

The MetadataWrapper from subwasm always wrapped around the RuntimeMetadata, fetching it from the RuntimeMetadataPrefixed which is defined as:

pub struct RuntimeMetadataPrefixed(pub u32, pub RuntimeMetadata);

This became an issue when exporting the .scale format since subxt expects a serialized RuntimeMetadataPrefixed and subwasm was providing the inner RuntimeMetadata, missing the u32 prefix.

This PR modifies the wrapper to allow exporting the prefix as well and make the scale export from subwasm compatible with subxt ❤️

related to #61

[chevdor]

@trusch beware: this PR fixes the scale format export but also the hex+scale and json+scale.

If you are using those last 2 formats, you may need to fix the usage as you now get an etra u32 at the beginning.