support for registering new ech certs dynamically

while discussing ech support for [haproxy when using aws-lc](https://github.com/haproxy/haproxy/issues/3333) one issue that was brought up that there is no API to manipulate the ECH certs of an ssl_ctx dynamically. is that part of your roadmap?

Start with the linked HAProxy issue 3333 and the discussion about ECH support with AWS-LC. Trace how ECH certificates are represented in an SSL context and define what a dynamic manipulation API would need to provide; done means an agreed API and coverage for registering new ECH certificates at runtime.

Written by the indexing model from the issue text.