docs: add an SDK roadmap for deferred features, open decisions, and codegen

[OmarAlJarrah]

Summary

Adds docs/roadmap.md, which organizes the open enhancement and design issues that aren't being handled as immediate fixes into a sequenced plan. Each entry links its tracking issue and records a recommended direction plus dependencies, so the larger work can be picked up in order rather than ad hoc.

Four tracks:

• Async parity — retry (Add an async retry step at the RETRY stage #31), bearer auth (Add an async bearer-auth step with background token refresh #32), and pagination (Async pagination support #34) for the async pipeline.
• Core capabilities — shared instrumentation emitter (Extract a shared instrumentation emitter from the sync and async steps #26), per-call options (Add a per-call options channel on the request context #27), per-phase Timeout (Add a per-phase Timeout value type (connect/read/write/request) #41), AutoCloseable SSE (Make the SSE stream an AutoCloseable Iterable bound to response close #35), multipart (Add multipart request body support #61), opt-in leak detector (Opt-in resource leak detector (log-only, no auto-close) #45).
• Decisions to make — URL model (Decide between a resolved java.net.URL and a deconstructed request URL model #29), QueryParams (Build a first-class QueryParams multimap and remove the QueryParam stub #28), pagination unification (Unify the two pagination stacks into one #30), value-equality util (Add a deep-array contentEquals/contentHashCode value-equality utility #49), a log-level confirmation (Confirm intended log level for the VirtualThreads executor close event #10), release automation (Decide on release automation before the first non-alpha tag #75).
• Code generation — the Design a dependency-free four-state JSON field model for generated types #50–Emit a Spring Boot starter per generated API #69 design set for a future KotlinPoet generator over this toolkit, with Design a dependency-free four-state JSON field model for generated types #50 (the four-state JSON field model) as the keystone and a suggested build order.

Documentation only — no code or build changes.