Add `unsafe_funding_transaction_generated` that wont check for fully signed inputs

[jbesraa]

In a channel opened in a payjoin scenario the inputs wont be fully signed before we return the psbt to the payjoin sender but we want to complete the FundingCreated & FundingSigned flow before returning the psbt.

unsafe_funding_transaction_generated should follow current function with a minor deletion of the assertion of signed inputs.

[tnull]

This was completed in #3024.