Payment API: Implement payer signing keys associated with an Identity

[stranger80]

We want to have a ability to detach the "Node Id" (identity) from the "Signer Address" - the key used to sign payment transactions (which is correlated with the wallet from which payments are made).

For an Identity (NodeId) we need to be able to specify Signer Addresses (multiple) with ability to indicate the "current" sender (so when a payment is scheduled via AcceptDebitNote/AcceptInvoice).

The Signer Addresses should be managed via Yagna CLI:

• AddSignerAddress(identity, signer)
• SetActiveSignerAddress(identity, signer)
• LockSIgnerAddress(signer)
• UnlockSignerAddress(signer)
• DeleteSignerAddress(signer)

This logic should be embedded in Payment API Implementation, ie for an appKey associated with AcceptInvoice request, the Node Id is determined, and this determines the active signer address to be selected by the Payment Driver for transaction signing. The signer address is passed to Identity API when passing transaction data for signing.

[tworec]

The signer address is passed to Identity API when passing transaction data for signing.

If I understand correctly the Payment Service is responsible for storing Signer Addresses.
Then it should be able to sign transactions by itself (ie. to have sign(identity, tx) operation`.

General scenario:

1. incoming REST API call with bearer key
2. bearer key gets translated into identity (via auth middleware)
3. Payment API impl gets active signer address for that identity and uses it for signing

This way sole responsibility for managing signer addresses is on Paymen service side. Identity service is not aware of it; nor any other service.