fix(google-ads): support flat extra format from connection form

[yuseok89]

This PR resolves a mismatch between the Google Ads connection form and the hook's expected input format. I discovered this issue while testing another PR(#61334). When users filled in the connection form (developer token, client ID, client secret, refresh token), the values were stored as flat keys in extra. However, the hook only read the nested google_ads_client structure, causing connection failures.

Before

Error when entering values via the connection form:

What the hook expected (nested format):

After

Supported connection form fields (flat format stored in extra):

How the connection was tested:

ads_hook = GoogleAdsHook(gcp_conn_id=..., google_ads_conn_id=...)
accessible = ads_hook.list_accessible_customers()  # Succeeds with flat extra

The hook now accepts both:

• Flat format (from connection form): developer_token, client_id, etc. at top level
• Nested format (legacy): google_ads_client with nested keys

Connection works when entering values via the connection form.

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