FR - Bring your own action file

### Why

Currently finishing the SPO scripts on the CLI. All is going pretty smooth, but today a showstopper popped up. With the change to require the constitution script hash for actions parameter-update and treasury-withdrawal, there is now a need to deal with the plutus script, collateral utxo(s), execution unit calculation, etc. ATM i think this is better handled by LightWallets on the Web.

Govtool could help out by allowing to bring your own action file.

### What

It would be nice to have an option to upload or copy/paste the content of an already made action file and import it into GovTool for further signing, processing, submit via a connected wallet. As this is currently mostly important for the parameter-update and treasury-withdrawal actions, it could also serve as a helper for all the other actions.

### How

Have not analyzed on how GovTool actually is set up, but looks like all is running as a JS env. So it should not be that hard to fulfil such a new feature imo. :-)