Add feign-graphql and feign-graphql-apt modules

[velo]

Summary

• New feign-graphql module: a GraphQL contract, encoder, and decoder for Feign
• New feign-graphql-apt module: an annotation processor that generates Java records from GraphQL schemas at compile time
• Given a @GraphqlSchema-annotated interface with @GraphqlQuery methods, the processor validates queries against the schema and generates input/result records, enums, and nested types

feign-graphql module

• GraphqlContract: extends Contract.Default, registers @GraphqlQuery annotation, stores query metadata (query string + variable name) in a map keyed by configKey
• GraphqlEncoder: wraps a delegate Encoder, builds {query, variables} JSON body from contract metadata. Also implements RequestInterceptor for no-variable queries
• GraphqlDecoder: unwraps GraphQL {data: {operationField: ...}} envelope. Supports pluggable delegate Decoder so users choose their JSON parser (Jackson, GSON, etc.)
• GraphqlErrorException: thrown when the response contains GraphQL errors
• No internal headers — metadata flows through configKey-based lookup

feign-graphql-apt module

• Annotations: @GraphqlSchema(path) on interface, @GraphqlQuery(query) on methods
• Validation: Queries are validated against the schema with line/column error reporting via Messager
• Type mapping: GraphQL scalars → Java types (Int→Integer, Float→Double, ID→String, etc.), [T]→List<T>, enums → Java enums
• Records: Generates Java records instead of POJOs — accessors use field() not getField()
• Nested generation: Selection sets produce result types with only selected fields; input types include all schema-defined fields
• Dependencies: graphql-java 22.4, javapoet 1.13.0, auto-service-annotations, compile-testing (test)
• Java 17 required (matches apt-test-generator)

Test plan

• 27 graphql tests pass (contract: 10, encoder: 5, client: 5, decoder: 7)
• 16 graphql-apt compile-testing tests pass
• mvn clean test -pl graphql,graphql-apt passes