internal/mcp: support prompts

Add support for 'prompts/list' and 'prompts/get', and registering
prompts on the server using AddPrompts. Add a 'MakePrompt' helper to
construct prompts from a prompt handler using reflection.

Change-Id: I479cb9c9b99313cca99640c1f5ea1939363759e6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/jsonschema/infer.go

@@ -142,6 +142,7 @@ func parseField(f reflect.StructField) (name string, required, include bool) {
 			}
 			name = props[0]
 		}
+		// TODO: support 'omitzero' as well.
 		required = !slices.Contains(props[1:], "omitempty")
 	}
 	return name, required, true