* Add ‘transaction_version’ to the signed transaction

This allows hardware wallets to know which transactions they can safely
sign.  To reduce transaction size, I reduced it to a ‘u8’ from a ‘u32’.

Fixes #5951.

* Restore transaction_version to a u32

* Fix comments

`transaction_version` is not part of a tx, but is still signed.

Co-authored-by: Bastian Köcher <[email protected]>

* Fix the test suite

I had forgotten to change the production of transactions in the test
code.

* Fix benchmarks

* Improve docs for `CheckTxVersion` in `frame_system`

Co-authored-by: André Silva <[email protected]>

* Remove spurious cast

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: André Silva <[email protected]>