Event-level reporting for buyers

[taymonbeal]

There's a bit of an open question as to how this API will work, since bidding_logic_url won't work exactly as described in the spec. Options include:

• Add a separate field that callers have to pass
• Have bidding_logic_url just do reporting and use trusted signals to choose the bidding algorithm
• Have bidding_logic_url do reporting and also make the choice of bidding algorithm in the global scope