Add new endpoints for CI API - incremental build log update

Related to gitlab-org/gitlab-ci-multi-runner#1155.

Add a new endpoints to CI API that will handle incremental update of build log.

/cc @ayufan

Would this fix the issue with build log exceeding limit of 4194304 bytes?

The UI pages are also incredibly slow when the log output is large. Chrome actually prompts to close them quite frequently due to being unresponsive.

@szechyjs The first iteration is to make the protocol more efficient from the Runner side, then we will introduce updates to rendering side. The limit should go away only if both are done.

Personally I would prioritize the rendering fixes, as it is currently unusable right now.

@szechyjs I'm sorry that you're having problems with the size. Consider reducing the size of the build log as a workaround for now. Reducing the server side load is something that is our priority to make GitLab.com more efficient. If you want to contribute to the client side rendering to get that earlier you are of course very welcome.

@szechyjs We have also some plans related to decreasing size of the build log. We are considering decreasing payload overhead data by doing some work in the frontend, but this will not happen quickly. For now you can try reducing build log using scripts, sending to /dev/null irrelevant parts of the output. /cc @ayufan

mentioned in merge request !3447 (merged)

Status changed to closed by commit 34eacc53

mentioned in commit 34eacc53

mentioned in commit 223c64f8