Add metadata to model versions

[avishniakov]

Describe changes

I implemented metadata support for Model Versions.
Usage:

# object-oriented outside of a pipeline
mv = ModelVersion(name=MODEL_NAME, version="latest")
mv.log_metadata({"foo": "bar"})
assert mv.metadata["foo"] == "bar" # True

# object-oriented inside a pipeline
@step(...)
def step_1():
    mv = get_step_context().model_version
    mv.log_metadata({"foo": "bar"})
    assert mv.metadata["foo"] == "bar" # True

# functional outside of a pipeline
log_model_version_metadata({"foo": "bar"}, model_name=MODEL_NAME, model_version="latest")
mv = ModelVersion(name=MODEL_NAME, version="latest")
assert mv.metadata["foo"] == "bar" # True

# functional inside a pipeline
@step(...)
def step_1():
    log_model_version_metadata({"foo": "bar"})
    mv = get_step_context().model_version
    assert mv.metadata["foo"] == "bar" # True

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)

[strickvl]

LGTM, though wondering again whether this needs a DB migration...

[fa9r]

Looks good to go 🙌

[github-actions]

E2E template updates in examples/e2e have been pushed.

[github-actions]

E2E template updates in examples/e2e have been pushed.

[github-actions]

E2E template updates in examples/e2e have been pushed.