Create new compiler helper method to return workflow dictionary only

[Tomcli]

@Tomcli

One thing comes to mind: I noticed that _write_workflow has the ability to simply return the yaml in-memory:

if package_path is None:
  return yaml_text

However, _create_and_write_workflow doesn't propagate that, as it doesn't return the _write_workflow's result. In the original KFP it wasn't a real issue, as the implementation of _create_and_write_workflow was as simple as:

workflow = self._create_workflow(
    pipeline_func,
    pipeline_name,
    pipeline_description,
    params_list,
    pipeline_conf)
self._write_workflow(workflow, package_path)
_validate_workflow(workflow)

so one could simply use _create_workflow if really needed. It's to as easy for kfp-tekton, _create_and_write_workflow being 60 lines.

Considering how compile is based on Argo KFP's version, it probably cannot be changed that way, but maybe another method that would return a yaml (or maybe a list of yamls, considering the possibility of multiple resources?) in-memory could be added? That could possibly result in an even bigger speed-up for some cases.

Originally posted by @Udiknedormin in #736 (comment)

We want to have a new method to return the workflow dictionary in Python memory to optimize the API response time when running the SDK as a service.