SignedExtension agnosticism

[seunlanlege]

To keep tesseract compatible with substrate runtimes in perpetuity, it needs to become agnostic to the signed extension requirements of transactions.

A good way to achieve this might be through a runtime api that acts as a signed extension factory. Creating and returning the signed extension data that the runtime expects.

This could then be exposed through the rpc allowing the relayer to query the signed extension to be included in extrinsics.