Support code annotation systems, such as Izuna

Proposal

The Izuna system is an excellent example of a way to provide an enormous amount of value to the display of code. It would be excellent if GitLab supported such systems.

A basic approach might be:

• add a concept of code annotation integrations (under ~"group::ecosystem" ), adding custom providers there as configuration on a per-project basis
• when displaying code (under groupsource code), send the locations of lexed identifiers to the configured server to receive annotations.