feat: Enable constructing GoogleServiceAccountClient with Credentials.from_service_account_info method

[gbretas]

Enable constructing GoogleServiceAccountClient with Credentials.from_service_account_info method

This PR implements the enhancement requested in issue #532.

Changes

• Added a new from_service_account_info class method to GoogleServiceAccountClient that accepts a service account info dictionary
• Updated class documentation to reflect the new functionality
• Added comprehensive unit tests for the new method
• Created an example file showing how to use this feature

Value

This enhancement allows users to create a GoogleServiceAccountClient using in-memory credentials rather than requiring a file path. This is particularly useful for:

• Cloud deployments where credentials might be stored in environment variables
• Containerized applications
• Applications using secret management tools
• Any situation where credentials aren't available as files

Usage

# Create client using in-memory credentials
service_account_info = {...}  # Dict containing service account credentials
oauth2_client = oauth2.GoogleServiceAccountClient.from_service_account_info(
    service_account_info, oauth2.GetAPIScope('ad_manager'))

The PR is backward compatible and follows the existing library patterns.