Auto model selection

[isidorn]

Creating this as a top level issue to track remaining work needed on the client side to make sure Auto shines:

Candidates to wrap up August release:

• ExP to control Auto visibility / default status #265233
• Disable Auto when preview features are disabled via GH policy #265902

Post-release (ideally in September):

• Transparently render the actual discount used in auto #265905
• Auto model fetching is slowing down first request by ~150ms (in Europe) #265903
• Ability to run client experiments to change what model Auto is resolved to #265904

[lramos15]

Pushing back as there are some open items still