Bundle signatures for later use - prevent importing multiple owner keys

[nicexe]

What is needed?

A way for an owner to sign for a safe transaction without the need of other owners to be loaded.
Other owners can load the signature for the transaction and add their own signature.
Once there are enough signatures for the transaction to be executed, an owner can submit the execution transaction to the network.

So the general idea is:

1. One of the owners creates a proposed safe transaction and passes it to the next owner.
2. The next owner signs for it and passes the transaction + their signature in a bundle.
3. Repeat 2 until enough signatures are gathered in the bundle (≥ threshold)
4. One of the owners submits the execution transaction for the proposed transaction along with the needed signatures.

(The owner that created a proposed safe transaction could also sign for it themselves as a step number 1.5)

Background

Currently, the owners need to share their keys on the same machine which defeats the purpose of a multisig wallet.

[Uxio0]

Sorry I missed this issue. As a workaround, you can use the tx-service mode to gather offchain signatures using our Safe Transaction Service

[fafrd]

Hello @Uxio0, in the light of the Safe API downtime today it's critical that this issue is addressed. Would you consider prioritizing this issue?

[Uxio0]

Hi, currently we don't have capacity for working on this, but we will accept and review any PR for this topic