bundle2: allow extensions to extend the getbundle request We want extensions to be able to request extra parts.

bundle2: add a way to add parts during a `getbundle` request We add a hook function to let extensions add parts to the bundle.

getbundle: pass arbitrary arguments all along the call chain The wireprotocol command accepts arbitrary arguments, we are now passing such arguments down the call chain.

bundle2: call a hook after the transaction is closed We call a dedicated hook right after closing the transaction. This will let people react to the transaction with all the information in hand. This hook is experimental and will not survive in future versions.