Support OpAMP server example in the opentelemetry-demo

[juandemanjon]

This PR aims to discuss how to port the server example to https://github.com/open-telemetry/opentelemetry-demo.

CC: @tigrannajaryan

opentelemetry-demo can be an excellent place to host the blocked PR #497 Add REST API to OpAMP Server Example. However, the server code is located in internal/examples, which makes it difficult to reference from the demo and could lead to unnecessary code duplication.

Is there any way to move the examples out of internal?

[juandemanjon]

This is the open issue at open-telemetry/opentelemetry-demo#2979