Migrate to buf schema v2 and move api related tasks to dedicated Makefile

[artemgavrilov]

PMM-7

Link to the Feature Build: SUBMODULES-0

This PR migrates buf configuration to v2. It allowed to move all api related tasks to dedicated Makefile.

P.S. I was trying to find a way to use some plugins only for part of the modules, but unfortunately it's not possible for now.

https://buf.build/docs/migration-guides/migrate-v2-config-files

Same for PMM v3: https://github.com/percona/pmm/pull/3178