feat: Add Metadata SDK support and samples for get method

[SinaChavoshi]

This feature is to add Metadata SDK support for get artifact, execution and context support. SDK sample updates will be

Note: Execution is currently an internal method, in a follow up PR we will be exposing it as an external method, the corresponding SDK samples will be covered in that PR.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/model-builder/experiment_tracking/get_artifact_with_uri_sample.py:18, tag aiplatform_sdk_get_artifact_with_uri_sample
• samples/model-builder/experiment_tracking/get_execution_sample.py:18, tag aiplatform_sdk_get_execution_sample

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