feat(aws-lambda): Add init & import spans #8085

Cold start's and module-level code aren't visible in the trace.

• function.aws.init: child of the first invocation, named {faas.name} init, only for a cold start a request waited for (on-demand and not proactive). Starts at process start (/proc/self/stat), else when the SDK loaded; ends when the first invocation starts.
• function.aws.import: the handler module import done by the layer