internal/mcp: keep an ordered list of feature keys

This change is necessary to support pagination which will be implemented
in subsequent CLs.

Change-Id: I8bc616a57463ea569dc7cadf1d4f06c69602ddd5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/674135
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

examples/sse/main.go

@@ -16,7 +16,7 @@ import (
 var httpAddr = flag.String("http", "", "use SSE HTTP at this address")
 
 type SayHiParams struct {
-	Name string `json:"name" mcp:"the name to say hi to"`
+	Name string `json:"name"`
 }
 
 func SayHi(ctx context.Context, cc *mcp.ServerSession, params *SayHiParams) ([]*mcp.Content, error) {