feat: add new optional fields and default definition name to config.yml

### Additions

* added new field `goals: [String!]!` to `DefinitionType` type
* added new field `authors: [String!]!` to `DefinitionType` type
* added new field `externalResources: [String!]!` to `DefinitionType` type
* added new field `exerciseType: String!` to `DefinitionType` type
* added new field `license: String!` to `DefinitionType` type
* added new field `language: String!` to `DefinitionType` type
* added new field `instructorRequired: Boolean!` to `DefinitionType` type
* added field `notes: ContentType!` to `ConfigType` type

Closes #525