New platform to bring your own runtime

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**Tell us about your request**
What do you want us to build?

For environments that require runtimes that Elastic Beanstalk doesn't support, we will provide a new platform with a supported Amazon Linux 2 host image. Customers can use this Amazon Linux 2 host image and install any self-managed runtime dependencies by using ebextensions. This allows customers to preserve any application runtime dependencies while using managed updates to keep their host current.

No repository files, tests, or implementation entry points are named. Start by reviewing the requested Amazon Linux 2 host image and ebextensions-based runtime installation requirements; done means delivering the proposed new platform while preserving managed host updates.

Written by the indexing model from the issue text.