[ci] Split gitlab-ci config to separate files

[alvicsam]

PR splits .gitlab-ci.yml to sub-files. I also renamed stages stage1,2,3,4 to weights,build, test, check, publish, zombienet, short-benchmarks.

Also I added an artificial job job-starter in the check stage. It triggers to longest jobs such as test-linux-stable and some builds. It makes pipeline faster.

Close https://github.com/paritytech/ci_cd/issues/613

[alvicsam]

bot rebase

[paritytech-processbot[bot]]

Rebased

[chevdor]

NOTE: This split causes troubles with miniexec (tool used in the release process). See https://github.com/s3krit/miniexec/issues/4 and https://github.com/s3krit/miniexec/issues/5