feat: Initial support for git submodules (#4605)

docs/usage/configuration-options.md

lib/config/definitions.ts

lib/datasource/git-submodules/index.ts

lib/datasource/index.ts

lib/manager/common.ts

lib/manager/git-submodules/artifacts.ts

lib/manager/git-submodules/extract.ts

lib/manager/git-submodules/index.ts

lib/manager/git-submodules/update.ts

lib/manager/index.ts

lib/platform/git/storage.ts

lib/versioning/git/index.ts

lib/workers/branch/get-updated.ts

lib/workers/repository/process/lookup/index.ts

renovate-schema.json

test/config/__snapshots__/validation.spec.ts.snap

test/datasource/git-submodules.spec.ts

...anager/git-submodules/__snapshots__/artifact.spec.ts.snap

...manager/git-submodules/__snapshots__/extract.spec.ts.snap

test/manager/git-submodules/_fixtures/.gitmodules.1