[UBP] Add placeholder implementation of FinalizeInvoice

[andrew-farries]

Description

Add a trivial 'noop' implementation of FinalizeInvoice to the billing RPC service.

Related Issue(s)

Part of #10937

How to test

• Unit test for the billing service
  or
• Port-forward to the usage service gRPC server:

kubectl port-forward svc/usage 9001:9001

• Use evans or grpcurl to hit the placeholder FinalizeInvoice RPC with an invoice id of e.g: 1234
• Tail the logs of deploy/usage and see:

{"level":"info","message":"Finalizing invoice for invoice \"1234\"","serviceContext":{"service":"usage","version":"commit-0aa8abfc2d3b27707a248d3ab285d3561202ceab"},"severity":"INFO","time":"2022-08-08T07:27:39Z"}

Release Notes

NONE

Documentation

Werft options:

• /werft with-preview
• /werft with-payment

[werft-gitpod-dev-com]

started the job as gitpod-build-af-placeholder-finalize-invoice-impl.4 because the annotations in the pull request description changed
(with .werft/ from main)

[laushinka]

Taking a look!

[laushinka]

This is so cool