Move InstanceMetadata concerns under AppConfig context

Description

As a follow-up from !164806 (comment 2087502138), in that MR we introduce the bounded context AppConfig. Let's move some of these non-bounded modules into this domain.

• app/models/instance_metadata.rb should become AppConfig::ServerInfo.
• app/models/instance_metadata/kas.rb should become AppConfig::KasInfo.
• app/graphql/types/metadata_type.rb should become Types::AppConfig::ServerInfoType.
• app/graphql/resolvers/metadata_resolver.rb should become Resolvers::AppConfig::ServerInfoResolver.

Bonus: After the GraphQL stuff is moved ☝ we should probably open another issue to deprecate the field metadata and rename it to serverInfo, so that there's some symmetry between API and implementation.