feature: support training for JumpStart model references as part of Curated Hub Phase 2

[Narrohag]

Issue #, if available:
N/A
Description of changes:

1. As part of the JumpStart Curated Hub Training Integration, changes have been made to add access configs to the CreateTrainingJob request for JumpStart model references. All model reference training channels will now include a HubAccessConfig field that includes the HubContentArn of the model reference. Gated model references will also include a ModelAccessConfig in the model channel that contains AcceptEula. The accept_eula field will be passed in through the .fit() method as a parameter. To ensure backwards compatibility, the user can continue to pass in accept_eula as an environment variable when initializing an Estimator. In either scenario, the CreateTrainingJob request will be built accordingly and all eula validation will happen in the training control plane.
2. Additionally, to handle a new field added to HubContentDocument called TrainingArtifactUri for training instance type variants, logic has been added to parse this and convert to/from TrainingArtifactKey from the model specs. This field provides instance type specific training artifact uris/keys to be used in place of the default training artifact if an instance type is specified.

Testing done:
The above changes have been tested with unit tests.

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