Add support to propose new protocol both on the client side and server side

[dbaeumer]

Currently proposing new LSP protocol is quite cumbersome since it requires publishing special version of the client and the server. This result in quite some adoption effort on both the implementation and the usage side.

We should enhance the libraries so that:

• new proposed API can be added easily
• one version of the library is publish
• clients of the library can opt into using the proposed API
• when the API gets final clients should be able to migrate to the final API with minimal effort

//cc @egamma

[dbaeumer]

Implemented for 3.4.0.