[functions][Issue:5350]Fix pulsar can't load the customized SerDe (#5357)

Fixes #5350

### Motivation

When using the `--output-serde-classname` option, `functionClassLoader` is not set correctly.
2 files changed
tree: 8a692711f5af7e661b1c6a6618f5dd91a5d7baff
  1. distribution/
  2. docker/
  3. tests/