Refactor LoadTestBuilder.groovy

LoadTestBuilder contains some sdk specific logic inside it's methods ("if"'s depending on sdk specific field names, sdk specific pipeline options format, etc).

How we could mitigate that: 
- Have common pojos representing all necessary parameters holding all options/job parameters in the same fields.
- Have one logic for all the operations and split it to more atomic operations. 
- Build options at the end to SDK specific representations.

Imported from Jira [BEAM-7539](https://issues.apache.org/jira/browse/BEAM-7539). Original Jira may contain additional context.
Reported by: ŁukaszG.

Start by reading LoadTestBuilder.groovy and the linked Jira issue BEAM-7539 for the missing context around its SDK-specific logic. Done means introducing common POJOs for the shared parameters, separating the operations into atomic steps, and building SDK-specific option representations only at the end.

Written by the indexing model from the issue text.