COWBIRD

[mbowiewilson]

This PR adds COWBIRD, a proposal for enabling ad-tech machine learning optimizations.

Here is a snippet of the proposal that summarizes it:

We propose the use of a paradigm in which the browser is responsible 
for evaluating tiny ad-tech models stored on-device. Later, the browser 
evaluates the gradients of these models and sends them to an aggregation 
service. After these gradients are aggregated, they are sent to the 
corresponding ad-tech company, where they can be used to privately 
improve model accuracy.

In this proposal, the browser acts as a federated learning platform,
allowing ad-tech companies to optimize toward customizable objectives
with customizable models and features.

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