refactor(enabled-managers): implement `custom.<customMgrName>` syntax (#24079)

docs/usage/configuration-options.md

...nfig/migrations/custom/enabled-managers-migration.spec.ts

lib/config/migrations/custom/enabled-managers-migration.ts

lib/config/validation.spec.ts

lib/config/validation.ts

lib/modules/manager/custom/index.spec.ts

lib/modules/manager/index.spec.ts

lib/modules/manager/index.ts

...ers/repository/extract/extract-fingerprint-config.spec.ts

lib/workers/repository/extract/extract-fingerprint-config.ts

lib/workers/repository/extract/index.spec.ts

lib/workers/repository/extract/index.ts