producer_worker: revert use of ValueGenerator::Generate()

[WillemKauf]

Using the modified ValueGenerator::Generate() function for each record in the producer_worker was a lot slower than just simply making an empty payload.

The following benchmarks were generated:

Benchmark_random_payload10-32           	 2445903	        485.2 ns/op
Benchmark_random_payload100-32           	  678870	         1773 ns/op
Benchmark_random_payload1000-32           	  105667	        13700 ns/op
Benchmark_random_payload1e4-32          	    9104	       126782 ns/op
Benchmark_random_payload1e5-32          	    1135	      1151425 ns/op
Benchmark_random_payload1e6-32          	     100	     13046547 ns/op
====================================================================================
Benchmark_empty_payload10-32            	79950942	        20.54 ns/op
Benchmark_empty_payload100-32            	24089101	        54.78 ns/op
Benchmark_empty_payload1000-32            	 3789248	        366.1 ns/op
Benchmark_empty_payload1e4-32           	  670096	         2059 ns/op
Benchmark_empty_payload1e5-32           	   58912	        23021 ns/op
Benchmark_empty_payload1e6-32           	    5600	       290045 ns/op

As can be seen, generating the random payload and ensuring it is UTF-8 valid is orders of magnitude slower than the empty payload.

Revert the use of Generate() here in place of the empty payload.

However, if the user has indicated they want to validate the latest key-value pair produced, generate a (value-{%018d}, offset) message in the record.

This does mean that message sizes that are less than 24 bytes are not honored if the validate-latest-values flag is passed.

[andrwng]

nit: the numbers would go well in the commit message too IMO, for git history spelunking

[WillemKauf]

Testing CI here: redpanda-data/redpanda#24558

[WillemKauf]

Redpanda CI is green.