Implement aggreagte partial sigs helper functions

[ekrembal]

Issue Description

We need to calculate sigHash and Tweak to aggregate partial sigs.
Implement util functions that calculates sighashes from deposit_outpoints, then aggreagtes the partial sigs. This function will be called from the backend itself and from flow tests

Tasks

Parts of the task:

• Impl aggreagte operator take partial sigs
• Impl aggreagte operator slash or take partial sigs
• Impl aggreagte move partial sigs

Deadline of the Issue

Monday