Add Supervised Fine Tuning Train Operator, Hook, Tests, Docs

[CYarros10]

This pull request adds the following:

SupervisedFineTuningHook: Hook for Google Cloud Vertex AI Supervised Fine Tuning APIs.
SupervisedFineTuningTrainOperator: Use the Google Cloud Supervised Fine Tuning API to create a tuning job.

About Model tuning: a crucial process in adapting Gemini to perform specific tasks with greater precision and accuracy. Model tuning works by providing a model with a training dataset that contains a set of examples of specific downstream tasks.

A sample DAG containing these operators could look like:
JSONL training data arrives in GCS >> GCSObjectExistenceSensor >> SupervisedFineTuningTrainOperator >> GenerativeModelGenerateContentOperator

[MaksYermak]

@CYarros10 What do you think about adding Links to this Operator and, maybe, for the previous operators related to generative AI? It is an example of code how it looks for PipelineJob https://github.com/apache/airflow/blob/main/airflow/providers/google/cloud/operators/vertex_ai/pipeline_job.py#L115 and https://github.com/apache/airflow/blob/main/airflow/providers/google/cloud/links/vertex_ai.py#L329

[CYarros10]

@CYarros10 What do you think about adding Links to this Operator and, maybe, for the previous operators related to generative AI? It is an example of code how it looks for PipelineJob https://github.com/apache/airflow/blob/main/airflow/providers/google/cloud/operators/vertex_ai/pipeline_job.py#L115 and https://github.com/apache/airflow/blob/main/airflow/providers/google/cloud/links/vertex_ai.py#L329

I think this is a great idea, I would love to do this but want to prioritize CountTokensAPI and EvaluationAPI as these could be part of a broader LLM pipeline with SupervisedTuningFineTrainOperator and GenerativeModelGenerateContentOperator. Will work on Links when I've completed those first!

[CYarros10]

Refactored the code in this PR to be included in airflow/airflow/providers/google/cloud/operators/vertex_ai/generative_model.py - to keep all generative AI / generative model operations in one place. as well as hooks, tests, docs, etc. Feel free to add thoughts @MaksYermak - thank you for the review!

[MaksYermak]

LGTM

[potiuk]

Nice one