Add support for Vertex AI authentication using JSON service account key

[Roisfaozi]

Background:

• Currently, the Anthropic Go SDK does not directly support authentication to Vertex AI using JSON service account keys, a common method for managing credentials in Google Cloud environments.
• This feature is essential to simplify integration with Vertex AI and provide more flexibility in credential management.

Goal:

• Add a new feature to the Anthropic Go SDK allowing users to:
• Specify the path to a JSON service account key file in the SDK configuration.
• Use the credentials in the file to authenticate to Vertex AI.
• Make authenticated API calls to Vertex AI.

Proposed Implementation:

• Option 1: New Configuration Function:
• Add a new configuration function to the SDK to specify the path to the JSON service account key file.
• This function will load the credentials and use them to create a Vertex AI client.
• Option 2: Additional Option in Client Creation Function:
• Add an optional parameter to the client creation function to accept the path to the JSON service account key file.
• Option 3: Environment Variable:
• Allow users to specify the path to the credential file via an environment variable.

Example Usage:

// Option 1: New configuration function
client, err := anthropic.NewClient(ctx, anthropic.WithVertexAICredentialsFile("/path/to/your/key.json"))

// Option 2: Additional option in client creation function
client, err := anthropic.NewClient(ctx, anthropic.WithVertexAICredentials(ctx, "/path/to/your/key.json"))