mcp,design: revert 'content' back to an interface type (#29)

After some experience with the flattened version of Content, we see that
it can easily lead to incorrect usage and is a harder API to read.
Therefore, this CL changes back to a design similar to what we had prior
to https://go.dev/cl/672415, though opting to promote content
unmarshalling to the protocol types that use it, which leads to an
overall simpler api.

internal/readme/client/client.go

@@ -40,7 +40,7 @@ func main() {
 		log.Fatal("tool failed")
 	}
 	for _, c := range res.Content {
-		log.Print(c.Text)
+		log.Print(c.(*mcp.TextContent).Text)
 	}
 }
 

internal/readme/server/server.go

@@ -18,7 +18,7 @@ type HiParams struct {
 
 func SayHi(ctx context.Context, cc *mcp.ServerSession, params *mcp.CallToolParamsFor[HiParams]) (*mcp.CallToolResultFor[any], error) {
 	return &mcp.CallToolResultFor[any]{
-		Content: []*mcp.ContentBlock{mcp.NewTextContent("Hi " + params.Name)},
+		Content: []mcp.Content{&mcp.TextContent{Text: "Hi " + params.Name}},
 	}, nil
 }