feat: helm requirements.yaml support (#3805)

docs/usage/configuration-options.md

lib/config/definitions.ts

lib/datasource/helm/index.js

lib/datasource/index.ts

lib/manager/helm-requirements/extract.ts

lib/manager/helm-requirements/index.ts

lib/manager/helm-requirements/update.ts

lib/manager/index.ts

renovate-schema.json

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

test/datasource/__snapshots__/helm.spec.ts.snap

test/datasource/helm.spec.ts

test/datasource/helm/_fixtures/index.yaml

...ager/helm-requirements/__snapshots__/extract.spec.ts.snap

...nager/helm-requirements/__snapshots__/update.spec.ts.snap

test/manager/helm-requirements/extract.spec.ts

test/manager/helm-requirements/update.spec.ts

...rkers/repository/extract/__snapshots__/index.spec.js.snap