Fixes CODY-3081
This is the first PR for the project [⏭️ Launch Cody API
Experimental](https://linear.app/sourcegraph/project/launch-cody-api-experimental-8fd5ec338bf4),
which falls under the umbrella of moving Cody's brains to the cloud.
Previously, there was no publicly available REST API for our customers
to interact with Cody. This is a frequently requested feature from
customers and prospects.
This PR adds a new `POST /api/v1/chat/completions` endpoint, which
should be compatible with existing OpenAI clients. The OpenAI API format
is increasingly becoming an industry standard so this seems like a good
first step towards exposing a stable publicly facing API for our
customers.
The goal is to add more Cody-specific APIs in the coming weeks to send
chat messages and reference context.
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## Test plan
See added test cases.
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## Changelog
* API: new publicly available `/api/v1/chat/completions` REST endpoint
that is compatible with OpenAI clients with some restrictions. The detailed list of restrictions will eventually be documented on sourcegraph.com/docs
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