feat: multi-round-trip request implementation (SEP-2322) (#950)

### Context

Design (`design/mrtr.md`) and implementation proposal for Multi
Round-Trip Requests (MRTR) per
[SEP-2322](de6d76fba3/seps/2322-MRTR.md).

### Changes

- Introduce `InputRequest` and `InputResponse` sealed interfaces with
corresponding `InputRequestMap` and `InputResponseMap` types for custom
map JSON codec implementation.
- Extend `CallToolParams`, `GetPromptParams`, and `ReadResourceParams`
with `InputResponses`, `RequestState` and an unexported `resultType`
("complete" or "input_required") for retry round-trips. The type is set
by the SDK based on input requests presence.
- Add client-side MRTR middleware enabled by default that automatically
fulfills input requests and retries.
- Added MRTROptions on ClientOptions to allow disabling the middleware
and configuring the number of retries.
- Add server-side MRTR middleware for backward compatibility:
transparently bridges input requests to direct
Elicit/CreateMessage/ListRoots calls for older clients

go.mod

@@ -15,5 +15,6 @@ require (
 
 require (
 	github.com/segmentio/asm v1.1.3 // indirect
+	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect
 )