Expose proposal answering via the Decidim GraphQL API

Expose proposal answering via the Decidim GraphQL API: a task in bird-critic-1.5-full-300 (Harbor dataset). Decidim's GraphQL API currently does not support answering proposals as a mutation. This PR adds a ProposalMutationType that wraps an individual proposal and exposes an answer mutation…

The task

Decidim's GraphQL API currently does not support answering proposals as a mutation. This PR adds a `ProposalMutationType` that wraps an individual proposal and exposes an `answer` mutation field, backed by a `ProposalAnswerType` that returns the answer state. Mutation resolution goes through the standard…

Part of kochsnow/bird-critic-1.5-full-300.