feat: add support for multiple client versions, change aiplatform from compat.V1BETA1 to compat.V1

[morgandu]

• add compat module

• centralize types and servcies (client) in compat for V1 and V1BETA1
• update all types in google.cloud.aiplatform modules to point to compat.types
• update all services (client) in google.cloud.aiplatform modules to point to compat.services

• In utils.py

• change the WrappedClient to an abstract ClientWithOverride class
• add (Service)ClientWithOverride for each AiPlatformService to predefine _default_version and _version_map
• add a TypeVar AiPlatformServiceClientWithOverride, and update TypeVar AiPlatformServiceClient

• In initializer.py

• add select_version to get_encryption_spec

• In each AiPlatformResourceNoun class

• change client_class to (Service)ClientWithOverride

• In BatchPredictionJob, Endpoint, and Model class

• add client version selection logic

• unit tests

• update existing tests to make sure all tests pass
• add unit tests to make sure AiPlatformServiceClientWithOverride works for V1BETA1 and V1
• point all types and services in tests.unit.aiplatform to V1
• add fixtures to support V1BETA1 tests for explanation related functionality
• make sure all tests pass (merging from fix: add v1 conversion value rule)

Fixes b/180711319 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/model-builder/create_and_import_dataset_image_sample.py:19, tag aiplatform_sdk_create_and_import_dataset_image_sample
• samples/model-builder/create_batch_prediction_job_sample.py:19, tag aiplatform_sdk_create_batch_prediction_job_sample
• samples/model-builder/create_training_pipeline_image_classification_sample.py:17, tag aiplatform_sdk_create_training_pipeline_image_classification_sample
• samples/model-builder/init_sample.py:20, tag aiplatform_sdk_init_sample

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[sasha-gitg]

This looks great! Thank you for all the hard work!

(FYI, need to run blacken for linting to pass)