feat: expose boot disk type and size for CustomTrainingJob, CustomPythonPackageTrainingJob, and CustomContainerTrainingJob

[morgandu]

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Fixes #<b/193822723> 🦕

• Change _MachineSpec to _WorkerPoolSpec
  • Add boot_disk_type and boot_disk_size_gb in _MachineSpec and _DistributedTrainingSpec
• Expose boot_disk_type and boot_disk_size_gb in run with default values of CustomTrainingJob, CustomPythonPackageTrainingJob, and CustomContainerTrainingJob
  • Add in boot_disk_type and boot_disk_size_gb in _CustomTrainingJob._prepare_and_validate_run
• Expose boot_disk_type and boot_disk_size_gb in CustomJob.from_local_script
  • Update TypeHint for command line arguments args to be passed to the Python task
• Modify unit tests to for default and overwrite boot disk config

[sasha-gitg]

LGTM for training_jobs.

jobs.CustomJob.from_local_script should also be updated: https://github.com/googleapis/python-aiplatform/blob/master/google/cloud/aiplatform/jobs.py#L1050

[sasha-gitg]

LGTM. Thanks Morgan!