[FR] Add multi-signature support (multi owner)

[rcmorano]

It would cool to enable users to perform multi-sig transactions. This is very possible using current approach which uses cardano-cli as backbone for handling transaction builds.

Multi-sig address creation workflow proposal:

• tx-builder request "verification key" from owners (we need a button somewhere for this)
• tx-builder generates script with atLeast/any/all requirements [0]
• tx-builder/any-owner submits script to the blockchain (pay for fees + minUtxO) [1]
• anyone can now send funds to the generated script address and they will be custodied by the script (multi-sig required to spend funds)

Multi-sig script-address spend workflow:

• tx-builder/any-owner creates a tx with a tx-in from script-address [2]
• tx-builder requests witness file from the required owners (could be this request sent via blockchain using metadata? and then witness files received via blockchain in the same way? ouroboros :o)
• tx-builder assembles the tx and submits it [3]

[0] https://github.com/input-output-hk/cardano-node/blob/master/doc/reference/simple-scripts.md#json-script-syntax
[1] https://github.com/input-output-hk/cardano-node/blob/master/doc/reference/simple-scripts.md#sending-ada-to-a-script-address
[2] https://github.com/input-output-hk/cardano-node/blob/master/doc/reference/simple-scripts.md#step-1---construct-the-tx-body
[3] https://github.com/input-output-hk/cardano-node/blob/master/doc/reference/simple-scripts.md#step-3---construct-and-submit-the-transaction