GAM REST API for private auctions and private auction deals

[martynaut]

internal ID: MONDE-304

Add GAM REST (Beta) API connection

New file: sroka/api/google_ad_manager/gam_rest_api.py

Adds a connection to the Google Ad Manager REST (Beta) API, structured to
match the existing SOAP API pattern in gam_api.py.

init_gam_rest_connection(network_code) — authenticates using the same
ad_manager.json service account key and network_code from config.ini already
used by the SOAP connection. No new config entries required.

get_resource_from_admanager(resource, filter_str, page_size, order_by,
network_code) — generic paginated fetcher, analogous to
get_service_data_from_admanager on the SOAP side. Uses a resource_map to
validate the resource name and resolve it to a REST path, handles
nextPageToken-based pagination automatically, and returns a DataFrame. Raises
ValueError for unsupported resources.

Currently supported resources:

• PrivateAuction - networks/{networkCode}/privateAuctions
• PrivateAuctionDeal - networks/{networkCode}/privateAuctionDeals

get_private_auctions_from_admanager and
get_private_auction_deals_from_admanager — convenience wrappers over the
generic function, one per resource.

Adding future resources requires only a new entry in resource_map and an
optional wrapper function.

requirements.txt

• Added google-auth>=2.0.0 as an explicit direct dependency. Previously it was
  only a transitive dependency (via google-auth-httplib2); the REST API uses it
  directly for AuthorizedSession and service_account.Credentials.