Semantic conventions for outgoing FaaS invocations are missing

[arminru]

We have defined certain FaaS resource attributes and Cloud resource attributes for FaaS invocations to describe themselves, but there is currently no semantic convention for clients to report the respective information about the invoked function on outgoing invocations.
These details can obviously not be reported by using the client's resource attributes and therefore corresponding span attributes are needed (e.g., a non-cloud client can invoke a Lambda function or a cloud-based client or even FaaS invocation can invoke a function on another cloud provider or region which will not match its own resource attributes).