Support --ce-override on ping and apiserver sources

[jchesterpivotal]

Feature request

All Source-ducktyped objects include a CloudEventOverride, which serializes as extensions. In kn, this is exposed as an option for source binding create and source binding update, but not for comparable commands on ping and apiserver command groups.

Also print the cloud event overrides extensions in describe output for sources if present.

The request is to add identical support to these other sources for consistency.

[navidshaikh]

/assign

[navidshaikh]

@jchesterpivotal : I've updated the issue description to also print the ce-overrides values in describe output if present.