Make `selectAssets` responsible for change addr assignment and extraInputScripts

[Anviking]

• [x] Embed the assignment of change address directly into selectAssets
• [x] Embed the lookup of extraInputScripts directly into selectAssets

Comments

• This enables us to remove the TxOuts corresponding to the TxIns in Selection
• This will in the future allow us to make TxUpdate take a Selection instead of the component extra{Inputs, ...}

Issue Number

ADP-3356