Remove create_new_model_version arg of ModelConfig

[avishniakov]

Describe changes

I simplified the ModelConfig interface to avoid explicitly passing around create_new_model_version.
New behavior expects that the user keeps version==None if the new unnamed version is needed, otherwise model version will be retrieved/created based on the version argument.
Also new ModelStages.LATEST was introduced in order to give the ability to fetch the LATEST model version.

[BREAKING CHANGE] Old configurations using create_new_model_version might lead to unpredicted results, but the change needed is simple:

ModelConfig(name="foo",create_new_model_version=True) -> ModelConfig(name="foo")
ModelConfig(name="foo", version="bar", create_new_model_version=True) -> ModelConfig(name="foo", version="bar")
ModelConfig(name="foo", version="bar", create_new_model_version=False) -> ModelConfig(name="foo", version="bar")
ModelConfig(name="foo", version=None, create_new_model_version=False) -> ModelConfig(name="foo", version=ModelStages.LATEST)

[BEHAVIOR CHANGE] If a signature like ModelConfig(name="foo", version="bar", create_new_model_version=True) was used, it prevented 2 parallel pipelines from running pointing to same model version, this is not the case anymore for named versions.

Tests updates mostly relate to signature updates, but also some tests were removed as no longer valid ones.

Pre-requisites

Please ensure you have done the following:

• I have read the CONTRIBUTING.md document.
• If my change requires a change to docs, I have updated the documentation accordingly.
• If I have added an integration, I have updated the integrations table and the corresponding website section.
• I have added tests to cover my changes.
• I have based my new branch on develop and the open PR is targeting develop. If your branch wasn't based on develop read Contribution guide on rebasing branch to develop.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)

[fa9r]

The main change looks good but I'm not a fan of the changes to the client, models, and zenstore methods. For these, fetching the latest version by default made sense and was more convenient to use with the version arg being optional. I'd suggest reverting these changes as indicated in the comments below, otherwise LGTM.

[fa9r]

what's this used for?

[avishniakov]

Here

[avishniakov]

@fa9r , tests passed, if you don't see any more changes needed here, I would love to merge this to proceed with renaming ticket.

[fa9r]

LGTM