[FEATURE] - Python SDK improved the signing function such that you can skip overwriting

[sirdeggen]

Summary

There is a flag added to the sign function which allows a dev to specify - only sign something which doesn't already have an unlockingScript set, or re-run sign on all inputs.

Motivation

Useful in sceanarios where you have a partially complete tx already, perhaps someone else signed it. Then you can add your own input and only sign it.

The feature is here in py-sdk and should be replicated in the ts-sdk unless there is a superior way to handle this.

Additional References

Default could be false such that no breaking changes are introduced.