Add a switch for Instant Apply to use short model for lower context prompts (#497)

* use enum for model

* add small proxy endpoint and use when needed

* extend endpoint to reduce duplication and imply dependance

* use experiment service

* rename small -> short

* fix model name

* fix type

* separate endpoint impls for upcoming header changes

* remove extra spaces

* Update baseline

* Full update

* Update

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Co-authored-by: Vritant Bhardwaj <vrtoku@gmail.com>
Co-authored-by: Vritant Bhardwaj <vrbhardw@microsoft.com>