gcp adapter | support for CloudFunctions gen2

[sebastiankonieczek]

Is your feature request related to a problem? Please describe.
GCP CloudFunctions gen2 use CloudEvents as event format when creating a trigger function.
With spring-cloud-function-adapter-gcp only the old RawBackgroundFunction interface is supported.
This is really annoying as one needs to stick to GCP CloudFunction gen1

See: https://cloud.google.com/functions/docs/concepts/version-comparison

Describe the solution you'd like
The FunktionInvoker class should also implement the CloudEventsFunction interface from com.google.cloud.functions:functions-framework-api and support conversion of the payload to CloudEvents.