feat(backend): avoid downcast and use ExecutionSpec if possible

[yhwang]

Description of your changes:
Limit the direct Workflow data struct access to common.util and use ExecutionSpec and ExecutionStatus in other places. Create a data struct to represent the node status information in ExecutionStatus and add an API to access this information.

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[yhwang]

/cc @chensun
ExecutionSpec and ExecutionStatus are abstract interfaces added and implemented before which remove the argo Workflow data struct dependency at the backend, except the interface implementation (workflow.go under common/util).
In this PR, the downcast in api_converter.go is removed and use ExecutionSpec instead. This makes the argo Workflow dependency resides in the common/util.

Edit
The downcast was in report_server.go.

[chensun]

@gkcalat can you help review this change to make sure that it's compatible with your status IR work? Thanks!

[gkcalat]

Sorry for the delay, @yhwang.
Thank you for catching this issue!

/lgtm
/approve

[google-oss-prow]

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This pull-request has been approved by: gkcalat

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[yhwang]

appreciate the review @gkcalat