New adapter: alliance_gravity

[danielbaud]

🔧 Type of changes

• [x] new bid adapter
• [ ] bid adapter update
• [ ] new feature
• [ ] new analytics adapter
• [ ] new module
• [ ] module update
• [ ] bugfix
• [ ] documentation
• [ ] configuration
• [ ] dependency update
• [ ] tech debt (test coverage, refactorings, etc.)

✨ What's the context?

We want to add our own adapter that we are also adding in the main (go) version

🔎 New Bid Adapter Checklist

• [ ] verify email contact works
• [ ] NO fully dynamic hostnames
• [ ] geographic host parameters are NOT required
• [ ] direct use of HTTP is prohibited - implement an existing Bidder interface that will do all the job
• [ ] if the ORTB is just forwarded to the endpoint, use the generic adapter - define the new adapter as the alias of the generic adapter
• [ ] cover an adapter configuration with an integration test

🧪 Test plan

How do you know the changes are safe to ship to production?

🏎 Quality check

• [ ] Are your changes following our code style guidelines?
• [ ] Are there any breaking changes in your code?
• [ ] Does your test coverage exceed 90%?
• [ ] Are there any erroneous console logs, debuggers or leftover code in your changes?

[osulzhenko]

@danielbaud any updates on this one?