feat(tools): Add GoogleManagedMcpToolset for simplified Google MCP auth

[yugborana]

Introduces GoogleManagedMcpToolset, a McpToolset subclass that automates ADC-based authentication and endpoint URL resolution for Google Managed MCP servers (e.g., BigQuery).

• Auto-refreshes OAuth2 tokens via google.auth on every request
• Maps product names to endpoint URLs (initially supports 'bigquery')
• Supports custom scopes and project ID overrides
• Updates bigquery_mcp sample to use the new simplified API
• Adds 16 unit tests covering init, auth, and parameter forwarding

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• Closes: Create a subclass of MCPToolset to simplify connections to Google Managed MCP servers #3897
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[yugborana]

@GWeale @wuliang229 @ankursharmas @sasha-gitg

[yugborana]

@llalitkumarrr

[rohityan]

Hi @yugborana , Thank you for your contribution! We appreciate you taking the time to submit this pull request.
Closing this PR here as it belongs to adk-python-community repo.
We highly recommend releasing the feature as a standalone package that we will then share through: https://google.github.io/adk-docs/integrations/