[Enhancement] eliminate type assertions in the switch case.[cloudevents.go]

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/eventmesh/issues?q=is%3Aissue) and found no similar issues.

### Enhancement Request

This issue is a subproject issue of eventmesh. the subproject address is:
https://github.com/apache/eventmesh-go
located at:
eventmesh-go/plugin/protocol/cloudevents/cloudevents.go
analysis and expalanation:
assigning the result of this type assertion to a variable (switch msg := msg.(type)) could eliminate type assertions in switch cases

### Describe the solution you'd like


### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

Start with eventmesh-go/plugin/protocol/cloudevents/cloudevents.go and inspect the switch that currently uses type assertions in its cases. Apply the issue's proposed switch-variable approach so the case logic no longer repeats those assertions. Done means the switch cases use the asserted value directly; the payload names no tests to run.

Written by the indexing model from the issue text.