Simplify the TxPayload trait a little

[jsdw]

Most custom implementations will not do static validation, and for these, there's no need to require that a pallet and call name can be handed back. This tweak to the trait signature makes that clear.

I also exposed a way to create a SubmittableExtrinsic without going through the previous steps, in case somebody has bytes for a pre-signed extrinsic that they'd like to submit.

In response to #637 (though I think their issue is unrelated, it'll help with this sort of thing)

[TarikGul]

LGTM 👍 , From what I see this PR replaces using call to abstract information with call.validation_details so that all information is in a single struct (which i definitely like).

[jsdw]

Yeah; I realised that the call and pallet name were only ever used in validation so wanted to make it clear that they didn't need implementing/werent used otherwise :)

(one advantage just that it's then more obvious that you can just impl TxPayload with some already-encoded call data and all it has to do is write that out to some vec when asked, and you don't need to worry about pallet/call name etc)