[aiplatform] Allow explicit project_id parameter when initializing api_client

Hi,

I noticed an issue when initializing the API client in aiplatform.
In [base.py#L568](https://github.com/googleapis/python-aiplatform/blob/main/google/cloud/aiplatform/base.py#L568), the `api_client` is initialized with only the `credentials object`, but without an explicit `project_id`:

```
self.api_client = self._instantiate_client(
location=self.location,
credentials=self.credentials,
appended_user_agent=appended_user_agent,
)
```

The problem is that the credentials object only carries the `quota project ID` (see the [google.auth.credentials reference](https://google-auth.readthedocs.io/en/stable/reference/google.auth.credentials.html)), not the actual project ID.

This makes it impossible to specify a different project ID for API requests other than the one used for quota billing.

## Expected behavior

It should be possible to initialize the api_client with an explicit project_id, separate from the quota_project_id in the credentials.

## Why this matters

Some users may want to:

* Use credentials tied to one project for billing/quota purposes
* Target a different project for Vertex AI resources

Currently, this scenario is not supported because the project_id cannot be overridden.

## Possible solution

Support a project argument when creating the client and pass it explicitly:

```
self.api_client = self._instantiate_client(
location=self.location,
credentials=self.credentials,
appended_user_agent=appended_user_agent,
project=project_id, # <-- new
)
```

This would align with other Google Cloud client libraries that allow explicit project configuration.

Thanks!

Start in google/cloud/aiplatform/base.py at the api_client initialization around line 568 and trace _instantiate_client to see how client arguments are propagated. Add support for an explicit project_id while keeping it separate from the credentials' quota project ID, then verify that API requests can target the explicitly configured project.

Rédigé par le modèle d'indexation à partir du texte de l'issue.