Fix sagemaker orchestrator and step operator env vars and other minor bugs

[stefannica]

Describe changes

This PR fixes the following issues:

• Sagemaker has a limit on the size of the environment variable values passed to processing steps (256) and estimator steps (512). We exceed the 256 limit with our API Tokens now. Fixed by splitting large env var values into multiple chunks and reconstructing them on the other side
• the AWS container registry still runs into errors if the local AWS credentials don't have access to check that the repository exists
• zenml orchestrator flavor describe sagemaker and zenml step-operator flavor describe sagemaker fail because the configuration includes attributes that have a Union type and that is not currently supported by the CLI

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)

[schustmi]

Would this environment variable change also be necessary for the Sagemaker Step Operator?

[strickvl]

Would this environment variable change also be necessary for the Sagemaker Step Operator?

I imagine it does. Step Operator use Estimator steps and Orchestrator uses Processor steps, but they both allow env variables to be passed in. And from what I understood, the limitation wasn't clearly documented on AWS so we'll just have to try and find out.

[strickvl]

This logic looks good, though would love to have some test(s) added since this added functionality is fairly fiddly.