feat(sdk): Add an API to return prepared workflow as python dicts.

[ScrapCodes]

Which issue is resolved by this Pull Request:
Resolves #737

Description of your changes:
Add an API (i.e. prepare_workflow) to return prepared workflow as python dicts.

Advantage of this API is, that is saves few microseconds spent in compiling python dict in yaml and store on disk,
because it returns the python dict in memory.

Disadvantage of the approach,

It will allow the end users to direct consume internal data-structure and they may tend to rely on its structure. In future, we do not guarantee that the fields or structure of field will not change in this dict.

Environment tested:

• Python Version (use python --version):
• Tekton Version (use tkn version):
• Kubernetes Version (use kubectl version):
• OS (e.g. from /etc/os-release):

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[Tomcli]

thanks @ScrapCodes, can you add a simple example on how to use this new function? You can just extend a new section in the kfp-user-guide over here https://github.com/kubeflow/kfp-tekton/tree/master/guides/kfp-user-guide

[Tomcli]

FYI @Udiknedormin we added a new function in the compiler class to get the pipelineruns and the related loop CRs. Let us know if you think it needs more improvement.

[Tomcli]

/lgtm
/approve

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