Hubble NixOS module (closes #682) (#693)

- feat(hubble): add nixosModule
- feat(hubble): add all options to module
- feat(hubble): make passing the `hasura-admin-secret` optional in
module
- feat(hubble): make indexers a json value
- fix(hubble): pkgs.lib instead of lib
- fix(hubble): add self arg to module
- fix(hubble): attrs instead of attrset
- fix(hubble): quote around indexers json
- fix(hubble): add type to example
- feat(hubble): improve types of indexers
- fix(hubble): use str type instead of string
- test(hubble): add another oneof
- fix(hubble): simplify module types