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.

examples/sse/main.go

@@ -21,8 +21,8 @@ type SayHiParams struct {
 
 func SayHi(ctx context.Context, cc *mcp.ServerSession, params *mcp.CallToolParamsFor[SayHiParams]) (*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
 }