Item Spec initialization parameters

[azriel91]

ItemSpecs may need to be initialized with parameters provided by the end user.

Thoughts:

1. Most likely the parameters should be flow specific, as we may not want to mandate the user to provide parameters for flows that they are not using. However, if one item spec is used in multiple flows, it also means re-entering those parameters each time a new flow is used. Need to work out a good trade off.

2. Actually it's better to request that all initialization parameters are specified up front and together. If we separate them by flow, then one may expect that publishing an artifact to artifact_repository_url will automatically allow it to be used in a subsequent flow that downloads the artifact. If the artifact_repository_urls are not consistent between flows, then it's a bad user experience / a surprise.

3. But then again, if we have a dev environment flow, and later a production environment flow, not all users will care about the initialization parameters of all flows.

4. So, we probably need both workspace specific, and flow specific parameters.

What we need:

• Parameters common to different flows. Defined by Workspace type parameter.
• Parameters specific to certain flows. Defined by ItemSpec associated type.