feature request: reuse group.id for kafka source

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Is your feature request related to a problem? Please describe.

in current impl, we new a group id every time a source actor is built, which leaves tons of useless group id.
RisingWave cannot clean them automatically because it uses admin API of kafka.

risingwave/src/connector/src/source/kafka/source/reader.rs

Lines 80 to 89 in ddfa85b

	config.set(
	"group.id",
	format!(
	"consumer-{}",
	SystemTime::now()
	.duration_since(UNIX_EPOCH)
	.unwrap()
	.as_micros()
	),
	);

The requirement is that two actors cannot share the same group.id in an ongoing job. So we can use {fragment id}-{actor id} as an identifier, which makes it global unique.

as actors do not change frequently

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response