Optimize load tests w/generic registrations

With Generic Registration available, a way to indicate the value would be via the load tests.

In particular the following tests should be optimized:

http://metrics.beam.apache.org/d/fK0U4JqWz/cogbk-load-tests?orgId=1&var-processingType=batch&var-sdk=go
http://metrics.beam.apache.org/d/WNzYt13Zk/combine-load-tests?orgId=1&var-processingType=batch&var-sdk=go
http://metrics.beam.apache.org/d/UYZ-oJ3Zk/gbk-load-tests?orgId=1&var-processingType=batch&var-sdk=go
http://metrics.beam.apache.org/d/MOi-kf3Zk/pardo-load-tests?orgId=1&var-processingType=batch&var-sdk=go
http://metrics.beam.apache.org/d/-E9aGlFGk/side-input-load-tests?orgId=1&var-processingType=batch&var-sdk=go

Benchmarks show reduce per call overhead by about ~300ns per parameter in the invoked user function, just like the code generated, and any improvement should be reflected by the load test graphs going forward.