gradle: stop gradelizing the entire repo

[postamar]

Stop gradelizing the entire repo. Instead, gradle should only be used for doing java things. Python connectors and odds-and-ends like octavia-cli don’t care about gradle.

• Stop including all connectors in settings.gradle. Instead, only include java connectors (test on src/main/java directory existence) and which are GA (test on contents of metadata.yml file)
• Get rid of SUB_BUILD env var
• Add new WITH_CONNECTOR env var to selectively include ‘:airbyte-integrations:connectors:$WITH_CONNECTOR’. Figure out a way to also include transitive dependencies to other connectors. It’s not that hard, airbyte-ci already does it, see ‘build_include’ in ‘gradle.py’.
• Have airbyte-ci set this WITH_CONNECTOR env_var.