accounts/external, signer/core: clef support for 2930-type txs

[holiman]

This PR adds support to use clef as a backend for signing 2930 type transcations.

-----------------------
WARN [03-26|14:47:56.832] Served account_signTransaction           reqid=4 t=1.856328178s             err="request denied"
--------- Transaction request-------------
to:    0x8A8eAFb1cf62BfBeb1741769DAE1a9dd47996192
from:     0x8A8eAFb1cf62BfBeb1741769DAE1a9dd47996192 [chksum ok]
value:    1 wei
gas:      0x84d0 (34000)
gasprice: 500 wei
nonce:    0x5 (5)
chainid:  0x796f6c6f763378
Accesslist
 0. 0x8A8eAFb1cf62BfBeb1741769DAE1a9dd47996192
   0. 0x79e6c92a69cc29f3faafb3d3cfe5b6ffaea50d4cecde1c0b03442a56f16f337a
 1. 0x8A8eAFb1cf62BfBeb1741769DAE1a9dd47996192
   0. 0x79e6c92a69cc29f3faafb3d3cfe5b6ffaea50d4cecde1c0b03442a56f16f337a

Request context:
	NA -> NA -> NA

Additional HTTP header data, provided by the external caller:
	User-Agent: ""
	Origin: ""
-------------------------------------------
Approve? [y/N]:
> n
-----------------------
WARN [03-26|14:48:09.235] Served account_signTransaction           reqid=5 t=2.223563371s             err="request denied"

However, there is a snag. The ChainId is implemented by both legacy and new type transactions, and for the old type, it's derived from the v. But when we approve it, it's not yet signed, so a legacy tx looks like this:

--------- Transaction request-------------
to:    0x8A8eAFb1cf62BfBeb1741769DAE1a9dd47996192
from:     0x8A8eAFb1cf62BfBeb1741769DAE1a9dd47996192 [chksum ok]
value:    1 wei
gas:      0x84d0 (34000)
gasprice: 500 wei
nonce:    0x5 (5)
chainid:  0x7fffffffffffffee

Request context:
	NA -> NA -> NA

Additional HTTP header data, provided by the external caller:
	User-Agent: ""
	Origin: ""
-------------------------------------------
Approve? [y/N]:
> n

And that's not great. If we have a types.Transaction, we don't really know if the ChainId represents the "User wants to sign for X" or "This tx is signed for X". cc @fjl any ideas?

[holiman]

Fixed now

[fjl]

I have tested this via eth_sendTransaction and it appears to work.