Add JSON stream progress writer

[felixfontein]

What I did
I added a JSON stream progress writer. This allows to write machine readable output to stderr, which makes it a lot easier to use Docker Compose from other tools, like Ansible.

Related issue
Fixes #10872.

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[felixfontein]

I still need to run make docs and commit the changes. Unfortunately it never comes remotely near to completion with the not so great internet connection I have on this train, so that has to wait until later...

[felixfontein]

Docs are updated, CI passes (checked in felixfontein#1), but while testing this (now that I was actually able to build this) I noticed that Docker Compose apparently expects Docker Buildx and Docker CLI itself to also have a JSON progress writer. I'm getting invalid progress mode json errors when it tries to pull images, and building also doesn't seem to work.

[felixfontein]

I eventually noticed that in case of errors, the error is always printed regularly and not via the progress API. I added code in d3b85f9 that in case --progress json it also formats the error as JSON.

[felixfontein]

This is now ready for review.

[felixfontein]

It probably makes sense to set logging to JSON (logrus.SetFormatter(&logrus.JSONFormatter{}), see https://github.com/sirupsen/logrus/blob/master/README.md) to avoid random non-JSON logging messages to show up on stderr.

[ndeloof]

you can use rawjson so buildkit will render raw json solver events and you get a clean json stream for all events

[felixfontein]

I created moby/buildkit#4769 since I think it would be better if the rawjson output would write one JSON object per line, instead of using multiple lines to show a more human-readable JSON record (that's not really that human readable anyway).