Update ADK doc according to issue #1292 - 7 - External Access Tokens

[adk-bot]

Add a section "Using External Access Tokens".
Explain that for Google credentials, you can configure external_access_token_key to instruct the tool to retrieve an existing access token from tool_context.state instead of performing authentication itself.
This is useful when the agent is invoked in an environment where the user is already authenticated (e.g., a frontend passing the token).
Example:

AuthCredential(
    auth_type=AuthCredentialTypes.GOOGLE_CREDENTIALS,
    google_credentials_config=GoogleCredentialsConfig(
        external_access_token_key="my_access_token"
    )
)

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