Return `txid` when broadcasting PSBT

[kyranjamie]

Currently, regardless of whether the broadcast flag is true or false, we return the PSBT hex in all cases.

Developers broadcasting PSBTs almost always need to know the txid.

In the case of broadcast: true, we should delay the response to the page until the transaction has broadcast successfully.

So:

• broadcast: true then return { hex: string; txid: string }
• broadcast: false then return { hex: string }

This should be typed as a discriminating boolean literal in @leather-wallet/types

[kyranjamie]

This has been requested by the Hermetica team

[markmhendrickson]

I presume we can label this effort small?